Answer:
Sentence D is an example of a a simple sentence.
Step-by-step explanation:
A simple sentence has just one independent clause. It has no dependent clauses. The difference between an independent clause and a dependent one is that the former can stand alone as a sentence as it represents a complete thought. In this case, the subject of the sentence is the NP "the death penalty" and the predicate is "should be abolished in the United States".