Answer:
The answer is indeed the conjunction "yet".
Explanation:
A conjunction is a word used to connect other words, phrases, or clauses inside a sentence. A coordinating conjunction, more specifically, connects parts that have the same importance inside a sentence. It doesn't establish a subordinating relationship between clauses, for example. Both clauses connected by a coordinating conjunction are equally important.
In the sentence we are analyzing here, we have two clauses with different ideas. The first one claims someone was just a child. The second one states he did a lot for other people. These two ideas are contrasting because, in general, we do not expect a child or a very young person to do a lot for others. To connect the two sentences and introduce the idea of contrast, the coordinating conjunction employed is "yet".
Let's study another example with "yet":
- He loved her too much, yet he broke up with her.
Both clauses are equally important, they both offer complete ideas that help the audience understand something about someone. However, they are contrasting ideas. We do not expect someone to break up with the person they love.