This question types requires us to use an answer choice that provides new information in order to reconcile two seemingly incompatible items/events.
Here the two incompatible items 1) In order for a species of large abalone shellfish to develop from a species of smaller ones, they must spend less energy on finding food and avoiding predators, and more on competition in mating. 2) The fossil record shows that a species of large abalone shellfish developed from a smaller one only after otters (a predator of abalone) began to dominate the waters in which abalone live.
The paradox is that how could this species of large abalone shellfish develop from a smaller one if it must spend less energy avoiding predators, yet a predator has become very prevalent in its waters.
That's where we will hopefully find the answer choice that provides information that will make these two items compatible.
Answer choice C states "otters also prey on the abalone's competitors for food and so indirectly make it easier for abalones to get food".
With this new piece of information, we see that the otter shave now resulted in abalone's spending less energy in finding food, which would explain how it is possible for this species to have developed even though the otters are dominating the waters.
Please let me know if you have any other questions!