Which of the following is a stream cipher that uses variable length key from 1 to 256 bytes?

The correct choice is a well-known stream cipher: RC4.

RC4 is famous for its simplicity and speed, particularly in software implementations, making it a popular choice for various applications, including SSL/TLS in web encryption. One of its notable characteristics is its support for variable-length keys, allowing keys from 1 byte up to 256 bytes in length. This flexibility in key length is important as it allows the user to choose a key size that meets their security requirements while also balancing performance.

In contrast, other options have different characteristics. For example, RC2 is a block cipher with a fixed block size and key length, typically up to 128 bits, which does not support variable-length keys like RC4 does. AES (Advanced Encryption Standard) is also a block cipher with fixed key lengths of 128, 192, or 256 bits but does not allow variable lengths beyond these specific sizes. The FISH encryption algorithm, while less commonly referenced, also does not align with stream ciphers in the same manner as RC4.

Thus, RC4's design allows it to accommodate a range of key lengths, making it suitable for various encryption contexts where flexibility is desired.