Explanation:

- Option 1: Declaration

- A declaration is a formal statement. It does not apply to summons under Section 91 of the CrPC since issuing a summons is not merely declarative.

- Option 2: Mandatory

- Mandatory means it is required by law. In the context of Section 91, it suggests that issuance of summons should occur in every case, which is not accurate.

- Option 3: Discretionary

- Discretionary implies that the court has the power to decide whether or not to issue a summons.

- The court exercises judgment based on the necessity and relevance of documents.

- Option 4: Either (a) or (c)

- This suggests it could be a declaration or discretionary.

- As established, a declaration does not fit, leaving only discretionary.

Correct Answer: Option 3 - Discretionary