(c) Assertion (A) is true, but Reason (R) is false.
The amount of dissolved oxygen in water is far less than the oxygen present in air, so aquatic organisms must breathe faster to meet their oxygen needs.
The Assertion is correct — aquatic organisms do breathe faster. However, the Reason is wrong because oxygen dissolved in water is actually much lower (not higher) than oxygen in air. This low availability of O₂ in water forces aquatic animals to increase their breathing rate. Since the Reason is factually incorrect, option (c) is the right choice.