Final answer:
Most modern block ciphers use a 128-bit block size, though they operate on 32-bit words during the encryption process to optimize performance.
Step-by-step explanation:
The question pertains to the block size used by modern block ciphers in cryptographic systems. The correct answer is B) 128-bit. While different ciphers may use varying block sizes, the most commonly adopted standard for modern encryption algorithms such as AES (Advanced Encryption Standard) is a 128-bit block size.
Despite the block size these ciphers often operate internally on smaller pieces of data, such as 32-bit words for each round of transformation during the encryption process. This is due to the design choices made to optimize the cipher's performance on a wide variety of hardware including those that natively support 32-bit operations.