(i can't underline it so it's in bold)
which sentence contains an underlined clause?
1. my (two large dogs happily) chased each other around the backyard.
2. because of their size, my dogs must be (fed three meals daily).
3. i wash my dogs (outside near) the garage once every two weeks.
4. after walking my dogs, (i sat down) to do my homework assignment.

4. After walking my dogs, (I SAT DOWN) to do my homework assignment.

Explanation:

To answer this question we need to know a definition of a clause.

A clause is a group of words that consists of a subject and a verb.  A clause is different from a phrase, as the phrase does not contain a subject, a verb, or a complete thought (in the kitchen, two big cats, etc).

1. My (TWO LARGE DOGS HAPPILY) chased each other around the backyard - not a clause.

These words in parentheses create a phrase, but not a clause. It doesn't have a complete thought or a verb.

2. Because of their size, my dogs must be (FED THREE MEALS DAILY) - not a clause.

These words in parentheses create a phrase, but not a clause. It doesn't have a complete and a subject.

3. I wash my dogs (OUTSIDE NEAR) the garage once every two weeks - not a clause.

This is a phrase; it doesn't have a subject, a verb, and a complete thought.

So, only sentence 4 is a clause. It has a subject, a verb, and it could stand alone as it has a complete thought.

I hope it helped you :)