Read the sentence.

Brendan is playing in the backyard.

This is an example of a

simple sentence.

compound sentence.

complex sentence.

compound-complex sentence.

The answer to your question would be that the sentence in question is a simple sentence. A simple sentence in grammar is understood as a sentence with only one main or independent clause and no dependent or subordinate clauses.

By comprising a subject and a predicate, this short syntactic entity intends to convey a complete idea or meanings of idea. In this case, the NP "Brendan" is the subject of the sentence and the VP "is playing in the backyard" is the predicate.

This is an example of a simple sentence.