Data in transit is information moving across network links, where it can potentially be intercepted by an attacker, so the concern is protecting its confidentiality during that journey.

The primary benefit of enabling encryption for data in transit is that it ensures that data is unreadable to attackers during transmission: the data is turned into ciphertext that cannot be understood without the correct key.

Even if an attacker captures the packets while they travel, they see only scrambled ciphertext, so the confidentiality of the information is preserved.

It does not protect against physical theft of devices, which is a matter of data-at-rest protection and device security, not in-transit encryption.

Encryption actually adds processing overhead for encrypting and decrypting, so it does not improve — and may slightly reduce — the raw speed of data transfer.

Preventing unauthorised users from accessing the network is the job of authentication and access-control mechanisms, not of encrypting data as it moves.

Hence the core purpose is keeping intercepted data confidential and unreadable in transit, making the correct answer It ensures that data is unreadable to attackers during transmission.