The correct answer is D. Twofish because the question explicitly describes a symmetric cipher that: (1) works on 128-bit blocks, (2) supports keys up to 256 bits, and (3) was a finalist in the AES selection process intended to replace DES. Twofish matches all of these characteristics. It is a symmetric block cipher designed by Bruce Schneier and colleagues, and it was one of the five AES finalists (along with Rijndael, Serpent, RC6, and MARS). Twofish uses a 128-bit block size and allows key sizes of 128, 192, or 256 bits, aligning precisely with the “up to 256-bit keys” clue.
Why the other choices do not fit: RC4 is a stream cipher, not a 128-bit block cipher, and it is not an AES finalist. AES itself (Rijndael) is the algorithm that ultimately won the AES competition; the prompt says the algorithm was a finalist, not necessarily the winner, and the description is intended to distinguish a finalist option from AES itself. Blowfish supports variable key lengths up to 448 bits but uses a 64-bit block size, not 128-bit blocks, and it was not an AES finalist (Twofish was designed as Blowfish’s successor with a larger block size to address modern requirements).
In practical terms, the block size and key size characteristics matter for security and performance: 128-bit blocks are standard for modern symmetric encryption, and 256-bit keys provide a high security margin for long-term protection of sensitive logs. The mention of AES finalization also signals that this algorithm belongs to the same generation of modern block ciphers designed with strong resistance to cryptanalysis and efficient software/hardware implementation.
Therefore, based on the stated properties and its AES finalist status, the symmetric encryption algorithm is Twofish.
