Jawaban:

a. Hammers, which are tools,

b. They are used to polish nails, which... (Note: "polish" may not be the correct verb to use here, as hammers are not typically used for polishing. It's possible that a different verb was intended, such as "pound" or "drive".)

Here are some alternative examples that may better illustrate the use of relative defining clauses:

The dog, which is brown and fluffy, chased after the ball.

The car, which was parked on the street, had a flat tire.

The book, which was written by Jane Austen, is a classic romance novel.