(b) faster because the amount of dissolved oxygen in water is fairly low.
Aquatic organisms must breathe faster to extract sufficient oxygen, as water contains much less dissolved oxygen than air.
The key concept is that dissolved oxygen in water is present in very small amounts compared to oxygen in air. To meet their oxygen needs, aquatic organisms (e.g., fish) must process a larger volume of water rapidly, making their breathing rate faster than that of terrestrial organisms. Option (d) is a distractor — while technically related, the reason given in the textbook focuses on the low amount of dissolved oxygen, not the limited capacity of water to dissolve air. Option (a) is wrong because it implies aquatic animals need more oxygen, which is not the stated reason.