This form of encryption algorithm is asymmetric key block cipher that is characterized by a 128-bit block size, and its key size can be up to 256 bits. Which among the following is this encryption alg

Twofish is a symmetric key block cipher and AES finalist designed by Bruce Schneier, featuring a fixed 128-bit block size and support for 128-, 192-, or 256-bit key lengths. Although the question incorrectly labels it 'asymmetric,' the defining technical characteristics - 128-bit block size with keys up to 256 bits - uniquely identify Twofish among the given options.

HMAC is a message authentication code algorithm used for data integrity verification, not a block cipher designed for confidentiality encryption.

IDEA (International Data Encryption Algorithm) uses a 64-bit block size with a fixed 128-bit key, matching neither the 128-bit block size nor the variable key length described in the question.

Blowfish uses a 64-bit block size and a variable key length from 32 to 448 bits, so its block size does not satisfy the 128-bit block requirement specified.