The File Transfer Activity can be used to decrypt a file on the SFTP server using a PGP key. This activity prepares the file for importing into Marketing Cloud. The other activities do not have the option to decrypt a file.

The issue of reduced deliverability tied to hitting spam traps during large email sends is directly related to List Hygiene. Spam traps are often email addresses that are specifically set up by blacklist providers or others to catch spam, and hitting them can severely impact a sender's reputation. Effective list hygiene practices, such as regularly cleaning the email list of inactive or invalid addresses and confirming the opt-in status of subscribers, help avoid these traps and maintain good deliverability.

When configuring an SFTP user in Marketing Cloud, the following considerations should be taken into account:
B). Minimum password length is 12 characters: Ensuring password security, Marketing Cloud requires SFTP passwords to have a minimum length of 12 characters.
D). Once created, the user will need to be activated in order to use the SFTP: After the SFTP user account is created, it must be activated by the admin before it can be used for file transfers.
Reference: Salesforce Help - Manage SFTP Users

The reason why the total contact count of 500,000 is less than the summed total of email subscribers, synchronized contacts, and mobile subscribers is D. There are overlapping contacts in each of the channels.
This overlap occurs when the same contact is counted across different channels (e.g., a contact might be both an email subscriber and a mobile subscriber), thus leading to a lower total contact count when duplicates are not counted more than once.
Reference: Salesforce Help - Contact Count Calculation

Explanation
One-to-One Relationship is the type of relationship that should be used to link demographic information to its contact model in Contact Builder. This type of relationship indicates that each record in the source data extension matches one and only one record in the target data extension based on a common attribute