Which one of the following options is used to determine the strength of a modern encryption algorithm?

The OSI layer at which encryption operates (e.g., Layer 2, 3, or 4) describes where in the network stack encryption is applied, not the mathematical strength of the algorithm itself.

Cipher block size affects how data is divided for processing and can influence certain attack modes (e.g., birthday attacks on 64-bit blocks), but it is not the primary determinant of overall algorithm strength.

The strength of a modern encryption algorithm is measured by its resistance to brute-force attacks, which is exponentially proportional to the key size - doubling the key length squares the number of possible keys. For example, a 256-bit key has 2^256 possible combinations, whereas a 128-bit key has 2^128. NIST guidelines recommend minimum key sizes (e.g., 128-bit for AES, 2048-bit for RSA) specifically because key size governs computational infeasibility of exhaustive search.

Message digest size relates to hash function output length and integrity checking, not to the strength of the encryption algorithm being used to protect the data.