RC4 is primarily known for which of the following features?

RC4 is primarily recognized for its use of byte-oriented operations combined with a variable key size. This characteristic allows RC4 to produce a stream of pseudo-random bits based on the input key, which can be of varying lengths, typically ranging from 40 to 2048 bits. The ability to accommodate different key sizes provides flexibility in terms of security; longer keys generally enhance the security of the encryption process.

The byte-oriented nature of RC4 means that it encrypts data one byte at a time rather than processing a block of data, which is characteristic of block ciphers. This stream cipher design helps in applications where low latency and efficient memory usage are important, such as in SSL/TLS protocols for secure communications.

The other options focus on features that do not relate to RC4's primary characteristics. Fixed key sizes are not a feature of RC4, as its variable key size is one of its defining traits. Additionally, block interleaved parity and disk mirroring techniques are unrelated concepts in the context of encryption methods and do not pertain to the capabilities or features of RC4. Thus, the correct identification of the byte-oriented operations with a variable key size encapsulates what RC4 is best known for in the field of cryptography.