Block cyphers is defined as :

The correct answer is: option 2: Which treats block of plain text as a whole

Concept:

A block cipher is a method of encrypting text (or binary data) in which a cryptographic key and algorithm are applied to a block of data (typically of fixed size, e.g., 64 or 128 bits) as a whole rather than bit-by-bit or byte-by-byte.

Block ciphers work by:

• Dividing the plaintext into fixed-size blocks
• Encrypting each block independently or using a chaining mode (like CBC, ECB, etc.)

Explanation of options:

• Option 1 – One that encrypts a digital data system one bit at a time: ❌ This describes a stream cipher, not a block cipher.
• Option 2 – Which treats block of plain text as a whole: ✅ Correct. This is the definition of a block cipher.
• Option 3 – Which treats complete file as a whole: ❌ Incorrect. Block ciphers operate on blocks, not entire files.
• Option 4 – Which treats segmented file: ❌ Vague and not accurate for block cipher terminology.

Hence, the correct answer is: option 2: Which treats block of plain text as a whole