Answer: it is D
Explanation: This is because a complex sentence contains a subordinate clause or clauses which means it has another part of the sentence that supports the first one so in this case "were made out of gold" supports "the prizes"
Subject- The longest, most boring day of his life
Predicate- ended with a thud.
Hey there!
This link is All About the topic sentence of a paragraph:
http://www.csun.edu/~hcpas003/topic.html
Hope it helps!