What element is NOT a variable in the RC5 algorithm?

In the context of the RC5 algorithm, the key features that define its operation include the key size, block size, and the number of rounds. These elements are fundamental to the algorithm's configuration and directly influence the security and performance of the encryption process.

Key size refers to the length of the key used for encryption, which can be variable and affects the strength of the encryption. Block size determines the size of the data blocks that are processed in each round of encryption, and it too can be variable within the algorithm's design. The number of rounds refers to how many iterations of encryption are applied to the data for added security and can also be adjusted.

On the other hand, the operating system is not a part of the RC5 algorithm itself but rather an environment where the algorithm might be implemented. The algorithm's characteristics do not depend on the operating system, making it an irrelevant factor when discussing elements that influence the encryption process. Thus, the correct identification of the element that is not a variable in the RC5 algorithm is indeed the operating system.