Answer

(iii) Both (a) and (b) — i.e. Data Link Layer and Transport Layer.

Explanation

Flow control means controlling the rate at which a sender transmits data so that a receiver is not overwhelmed.

Flow control can be performed at more than one layer, but the mechanism and scope differ:

1. Data Link Layer: Provides hop-to-hop flow control between directly connected devices. Protocols at this layer can regulate the amount of data sent over a particular link.

2. Transport Layer: Provides end-to-end flow control between communicating processes. For example, TCP uses a receiver window (rwnd) so that a sender does not transmit more data than the receiving host can buffer.

Therefore, both the Data Link and Transport layers can perform flow-control functions, but at different scopes.

Exam point

Data Link = hop-to-hop flow control

Transport = end-to-end flow control

Final answer: (iii) Both (a) and (b).