Answer:
Compound sentence.
Step-by-step explanation:
A compound sentence is a sentence that contains at least two independent clauses. Moreover, the independent clauses are joined by a comma, or one of the coordinating conjunctions FANBOYS- for, and, nor, but, or, yet, so.
The given sentence contains two independent clauses - "David forgot his book" and "he had to sit there and do nothing". Moreover, the use of the coordinating conjunction "so" along with the comma before it makes the whole sentence a compound sentence.
Thus, the correct answer is a compound sentence.