The correct answer is C. 51% Attack.
A 51% attack occurs when one entity or coordinated group controls the majority of mining, validation, or computational power in a blockchain network. With majority control, the attacker can influence block creation, reorganize recent blocks, reverse transactions, and perform double spending.
CEH-aligned blockchain material explains that blockchain relies on distributed validation and that more than half of the participating systems must agree before a block is accepted into the chain . The scenario states that a single coordinated entity controlled a dominant share of computational power and used that influence to replace confirmed transactions with alternate versions, which is the core behavior of a 51% attack.
Option A. Eclipse Attack is incorrect because an eclipse attack isolates a victim node from the honest network and controls the information it sees.
Option B. Finney Attack is incorrect because a Finney attack is a specific double-spend technique involving pre-mined blocks, usually by a miner, but the scenario emphasizes majority control of validation power.
Option D. DeFi Sandwich Attack is incorrect because sandwich attacks involve transaction ordering/front- running around a victim's trade, not block reorganization through majority hash power.
Option C. 51% Attack is correct because disproportionate control over block creation enabled transaction reversal and double spending.
Therefore, the best answer is C. 51% Attack.

Webhooks are one of a few ways internet applications will communicate with one another.
It allows you to send real-time data from one application to another whenever a given event happens.
For example, let's say you've created an application using the Foursquare API that tracks when people check into your restaurant. You ideally wish to be able to greet customers by name and provide a complimentary drink when they check in.
What a webhook will is notify you any time someone checks in, therefore you'd be able to run any processes that you simply had in your application once this event is triggered.
The data is then sent over the web from the application wherever the event originally occurred, to the receiving application that handles the data.
Here's a visual representation of what that looks like:

A webhook url is provided by the receiving application, and acts as a phone number that the other application will call once an event happens.
Only it's more complicated than a phone number, because data about the event is shipped to the webhook url in either JSON or XML format. this is known as the "payload." Here's an example of what a webhook url looks like with the payload it's carrying:

What are Webhooks? Webhooks are user-defined HTTP callback or push APIs that are raised based on events triggered, such as comment received on a post and pushing code to the registry. A webhook allows an application to update other applications with the latest information. Once invoked, it supplies data to the other applications, which means that users instantly receive real-time information. Webhooks are sometimes called
"Reverse APIs" as they provide what is required for API specification, and the developer should create an API to use a webhook. A webhook is an API concept that is also used to send text messages and notifications to mobile numbers or email addresses from an application when a specific event is triggered. For instance, if you search for something in the online store and the required item is out of stock, you click on the "Notify me" bar to get an alert from the application when that item is available for purchase. These notifications from the applications are usually sent through webhooks.

The attacker is observing and capturing network traffic without altering it or interacting with the systems, which is characteristic of a passive attack.

A digital certificate, issued by a trusted Certificate Authority (CA), binds a public key to the identity of an entity (e.g., user, organization). This provides a means to validate ownership of the public key.
CEH v13 Reference:
Module 17: Cryptography
"A digital certificate ensures the authenticity of the public key through a trusted third party known as a Certificate Authority."
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