Which of the following encryption types is primarily focused on securing data during transit?

The selection of Secure Sockets Layer (SSL) as the correct answer is rooted in its primary function of protecting data as it travels across networks. SSL is a protocol used to establish a secure, encrypted connection between a client (such as a web browser) and a server, ensuring that any information exchanged during this connection remains confidential and tamper-proof. By encrypting the data packets that are sent over the internet, SSL helps to prevent unauthorized access and eavesdropping, effectively securing sensitive information during transit.

In contrast, whole-instance encryption secures an entire virtual machine or instance, which typically pertains to data at rest rather than transit. Data encryption at rest deals with securing stored data on disk drives or databases, focusing on protecting data when it is not actively being moved or processed. Application-level encryption typically involves encrypting data within an application itself but does not specifically address how data is transferred across networks.

Each of these alternatives serves vital security functions, but none are specifically aimed at protecting data during its transmission as SSL does, which solidifies its position as the best answer to the question.