What is the structure of the sentence?
the young boy’s toy was lost, so later his mother searched and found the toy under a blanket.
a. compound-complex
b. complex
c. simple
d. compound

The correct answer is D, because there are three independent clauses in this sentence, all of the clauses are independent. A means that there is at least one dependent clause, which is not the case here, and B means the same. C mean that there is only one independent clause in the sentence.