Which equation is used to encrypt a message in RSA?

The correct equation used to encrypt a message in RSA is based on a mathematical operation that is fundamental to the RSA encryption process. In RSA, the encryption of a plaintext message is performed using the formula:

C = M^e % n

Here, C represents the ciphertext, M is the plaintext message that needs to be encrypted, e is the public exponent, and n is the modulus, which is the product of two prime numbers used in key generation.

In the context of the provided choices, the equation given as the correct answer can be interpreted as relating to the modular exponentiation that takes place during RSA encryption. The equation "Me % n" aligns with this process because it signifies that the plaintext message M is raised to the power of the encryption key (or exponent) and then taken modulo n. Thus, without explicitly stating the exponent, the essential nature of applying the public key operation to the message is captured here.

While other options might represent various steps in the encryption or decryption process, the option that directly pertains to the fundamental operation of encrypting a message with RSA is the correct choice.