Which of the following is a valid AES key size?

• A. 64 bits
• B. 320 bits
• C. 128 bits
• D. 512 bits

Answer: (C)

AES uses only specific key lengths: 128, 192, or 256 bits. Among the options, 128 bits is valid because it is one of the official AES key sizes. The other values aren’t defined for AES: a 64-bit key is too short for modern AES security and is not an AES key length, while 320 bits and 512 bits are not standard AES key lengths. So 128 bits is the correct choice. (Note: the different key lengths correspond to different numbers of rounds—10 rounds for 128-bit keys, 12 for 192-bit, and 14 for 256-bit—affecting security and performance.)