Final answer:
The underlined group of words is called an independent clause.
Step-by-step explanation:
The underlined group of words in the sentence 'she wrote the letter, and he gave the letter to the teacher' is called an independent clause. An independent clause is a group of words that contains a subject and a verb and can stand alone as a complete sentence. In this sentence, 'she wrote the letter' and 'he gave the letter to the teacher' are both independent clauses.