In a public key infrastructure, trust in a certificate ultimately depends on the relying party's trust anchor set-typically the root CA certificates preinstalled in a customer's browser/OS trust store. For customers to accept the company's certificate as legitimate, the buyer (customer) must trust the root CA public key (or an intermediate chained to it) so they can validate the certificate chain and signatures. The seller (the company) also must trust and rely on the root CA public key to build and present a valid chain and to make operational decisions based on that CA's issuance and revocation mechanisms; practically, the seller selects a CA whose root is widely trusted by customers. The cloud provider's trust is not what makes the certificate valid to customers; the provider may terminate TLS or pass traffic through, but customer validation is based on the chain to a trusted root. Government agencies like the FTC are not part of the cryptographic trust path for TLS certificate validation.
Therefore, among the given options, the correct pairing is the seller and the buyer, reflecting both the issuer selection/usage by the company and the relying-party validation by customers.

A Caesar cipher is a classic monoalphabetic substitution cipher where each plaintext letter is replaced by a letter a fixed number of positions away in the alphabet. For example, with a shift of 3, A becomes D, B becomes E, and so on, wrapping around at the end (X#A, Y#B, Z#C). This "fixed shift" is the entire key: both sender and receiver must know the shift value to encrypt and decrypt. Decryption simply shifts letters back by the same amount. The Caesar cipher illustrates foundational cryptographic ideas: key-based transformation, reversible mapping, and the importance of key space size. Because the key space is tiny (only 25 meaningful shifts in the Latin alphabet), it is easily broken by brute force. It is also vulnerable to frequency analysis because letter frequency patterns in the ciphertext resemble those of the plaintext, just relabeled. While historically important for introducing substitution concepts, it provides no meaningful security by modern standards. The defining operation is the fixed positional shift, which directly matches option D.

CTR mode turns a block cipher (like AES) into a stream-like construction by generating a keystream from successive encryptions of a changing input block. Specifically, CTR forms input blocks using a nonce (unique per message) combined with an increasing counter. Each nonce||counter block is encrypted with the block cipher under the shared key, producing a pseudorandom output block. That output is then XORed with plaintext to yield ciphertext (and XORed with ciphertext to recover plaintext). This design enables parallelization (blocks can be generated independently), efficient random access decryption, and avoids chaining dependencies seen in modes like CBC. Option B describes CFB-like behavior; option C describes ECB; option D describes CBC. CTR's security critically depends on never reusing the same nonce/counter sequence with the same key, because reuse would repeat keystream blocks and expose plaintext relationships. Therefore, the correct description is that CTR converts the block cipher into a stream cipher using a counter value and a nonce.

3DES (Triple DES) applies the DES block cipher three times to increase effective security, and its commonly cited valid key sizes correspond to how many independent DES keys are used. Two-key
3DES uses two 56-bit DES keys (K1 and K2) in an EDE sequence (Encrypt with K1, Decrypt with K2, Encrypt with K1), yielding 112 bits of keying material (ignoring parity bits). Three-key 3DES uses three independent 56-bit keys for a total of 168 bits of keying material, but that option is not listed here.
A 56-bit key corresponds to single DES, not 3DES. 128-bit is associated with AES, not 3DES. 2,048-bit is typical for RSA keys, not symmetric ciphers. Therefore, among the choices provided, 112-bit is a valid 3DES key size. While 3DES is now deprecated for many uses due to its 64-bit block size and performance limitations, understanding its keying options remains important for legacy system assessment.

Disk/storage encryption protects data at rest by encrypting the contents of a drive so it remains unreadable without the correct authentication and keys. BitLocker (commonly on Windows) and FileVault (commonly on macOS) are well-known software-based full-disk encryption solutions integrated into their operating systems.
They encrypt sectors on disk and typically tie key protection to user credentials and, where available, hardware features such as a TPM or secure enclave to reduce key extraction risk. A VPN encrypts network traffic in transit, not disk storage. An HSM is specialized hardware used to generate, store, and protect cryptographic keys and perform crypto operations; it is not a disk encryption product itself. USB encryption hardware refers to hardware-encrypted removable media, not a software solution for a system disk. Therefore, the correct example of a software encryption solution for disk storage is BitLocker and FileVault.