What type of sentence is made up of two simple sentences joined with a and a coordinating conjunction?

The type of sentence made up of two simple sentences joined with a comma and a coordinating conjunction is: a compound sentence.

Explanation:

A simple sentence is constituted of a single independent clause. It has a subject and a predicate, and it expresses a complete thought. When we put two simple sentences together, joining them with a comma and a coordinating conjunction, we have what is called a compound sentence. An example of a compound sentence is:

Simple: I love chocolate ice cream.

Simple: My boyfriend loves strawberry ice cream.

Compound: I love chocolate ice cream, but my boyfriend loves strawberry ice cream.