Which of the following are used to generate a message digest by the network…

Note on labels: option (S) refers to MD5 (the Message-Digest Algorithm 5); it is sometimes mis-typed as "MDS".

Concept

A message digest is a fixed-length fingerprint of data produced by a cryptographic hash function. A hash function is one-way (you cannot reverse the digest back to the input) and is used for integrity - checking that data has not been altered. This is fundamentally different from encryption, whose job is confidentiality (hiding data so it can later be recovered with a key). So the question is really asking: which of these algorithms are hash functions, not ciphers?

Applying it to each algorithm

1. RSA - an asymmetric (public-key) cipher used for encryption and digital signatures. It transforms data reversibly with a key pair, so it is a cipher, not a digest generator.

2. SHA-1 - a cryptographic hash function that takes any input and outputs a 160-bit message digest. This is exactly a digest generator.

3. DES - a symmetric block cipher used to encrypt plaintext into ciphertext for confidentiality. Reversible with a key, so it is a cipher, not a digest generator.

4. MD5 - a cryptographic hash function that outputs a 128-bit message digest. Another digest generator.

Contrast (algorithm vs role)

Result

The message-digest generators are SHA-1 (Q) and MD5 (S), so the correct choice is Q and S only. Note: although SHA-1 and MD5 are today considered cryptographically weak (vulnerable to collision attacks), they are still by definition message-digest algorithms, which is what the question asks about.