The structure of above sentence is complex.
Answer: Option A
Step-by-step explanation:
When a sentence consists one independent clause and one or more subordinate clause is said to be a complex sentence. In the above sentence, ''Sheila stopped to help the puppy'' is the independent clause which can form a simple sentence by itself. ''Although it was raining and who had fallen into the deep hole'' are the dependent clauses that does not form a simple sentence by itself.
These two clauses connected by relative pronouns or adverbs are the examples of complex sentences. We can combine them together to get a complex sentence.