Which sentence contains an intransitive verb?

An intransitive verb doesn't require a direct object. A cannot be correct because, Ian sharpened what? His hockey skates. So that is the direct object, therefore the verb is transitive. C and D are the same case.
The correct answer is B, because the verb is intransitive as it doesn't need an object. You cannot answer - Carla skates what? No, it is just Carla skates.

Delegates: representatives who cast votes at a political party's national convention. binding: if a primary or caucus is “binding” that means the delegates won in that contest are legally bound to vote for a particular candidate at the parties national convention.a primary is a preliminary election used to determine a party's nominee for a specific office. the winner of the party's caucus, or primary, wins the party nomination for that state. then the delegates meet at their party's national party convention, which is a formal gathering of all party delegates from all states.