Which of the following types of cryptography algorithms is a symmetric key cipher which operates on fixed-length groups of bits, termed blocks, with an unvarying transformation?

RC2 is a specific variable key-size block cipher algorithm developed by Ron Rivest, not the general category name for the type of cipher described in the question.

A stream cipher encrypts data one bit or byte at a time continuously rather than in fixed-length blocks, making it the conceptual opposite of what the question describes.

A block cipher is defined as a symmetric key cipher that divides plaintext into fixed-length groups of bits - called blocks - and applies a consistent, deterministic transformation to each block using a shared secret key. This definition precisely matches the question's description of 'fixed-length groups of bits' with an 'unvarying transformation.' Common examples include AES and DES, and block ciphers are the foundational category from which many encryption modes of operation (CBC, ECB, CTR) are derived.

Triple DES (3DES) is a specific symmetric encryption algorithm that applies DES three times to each block, making it an example of a block cipher rather than the general classification being asked about.