What is a modern symmetric block cipher using 128-bit blocks?

• A. DES
• B. AES
• C. CBC Mode
• D. CFB Mode

Answer: (B)

The essential idea here is a modern symmetric block cipher that processes data in 128-bit blocks. The only option that fits is AES, which uses fixed 128-bit blocks and was designed as a robust successor to older standards. AES (Rijndael) operates on 16-byte blocks and supports key sizes of 128, 192, or 256 bits, making it widely adopted for both performance and security in modern systems.

The other choices don’t fit. DES is an older cipher that uses 64-bit blocks and a relatively small 56-bit key, which makes it insecure by today’s standards. CBC and CFB are modes of operation, not block ciphers themselves; they specify how to use a block cipher like AES to encrypt data, but they are not the encryption algorithm on their own.