What is the mathematical expression for symmetric decryption?

The correct answer reflects the relationship between the plaintext, ciphertext, and the encryption key in the context of symmetric encryption. In symmetric encryption, both the encryption and decryption processes use the same key.

The mathematical expression for symmetric decryption is typically represented as:

P = C(d, k)

In this expression, 'P' represents the plaintext, 'C' is the ciphertext, 'd' refers to the decryption process, and 'k' is the symmetric key being used. This indicates that you retrieve the plaintext (P) by applying the decryption process (C) on the ciphertext (C) using the key (k).

The other expressions do not accurately illustrate the symmetric decryption process. For instance, the first choice suggests an incorrect usage of symbols and does not denote a decryption function. Similarly, options involving 'E' denote encryption operations, which do not align with the decryption process being asked about.