Hi ndimas,
Thanks for the question! Generally, it will help us target our answer to make it more helpful to you if you provide an explanation of how you approached the question, which answer you chose, and why.
Generally, though, this question uses sufficient/necessary reasoning, so you'll want to start by diagramming.
Premise: large species developed from small species
less energy on food/avoiding predators AND more competition in mating
Given that, you need to explain why large abalone developed from small ones only after otters appeared.
The correct answer is going to tell you that otters = either less energy on food/avoiding predators OR more competition in mating OR both.
C does that: It tells you that otters mean less competition for food, which would help explain why otters led to the development of a larger species of abalone from a smaller species.
Hope that helps!