This question is tricky as it's a different format then we typically see in logical reasoning questions. You're looking for the answer choice which creates a conclusion that follows logically from the premises provided. Here's a breakdown of the argument:
Premise: Only students genuinely curious about a topic can successfully learn it. Premise: This is because they the find the satisfaction of their curiosity gratifying and appreciate the rewards of the learning process. Premise: But, almost no child starts with enough curiosity to learn all that they need to. Conclusion: So, a teachers job is ???.
Try to fill in the conclusion on your own before turning to the answer choices. The premises tell us that children can only learn if curious, but they aren't curious enough on their own. So, a teacher then has to help them be more curious.
Answer (B) is incorrect because it goes off topic. The argument does not mention the use of rewards (outside of the intrinsic rewards inherent in the process). There is no support in the argument for the idea of rewards.
Answer (A) is correct because it fills in the gap regarding curiosity. If a child needs to be curious to learn but isn't curious enough on their own, a teacher must not help them only in the final step of learning (aka satisfying their curiosity) but also in raising their curiosity so that they will successfully learn.