Which of the following is a well-defined set?

1. The set of tall students in a class

2. The set of good books

3. The set of prime numbers less than 20

4. The set of beautiful flowers

A set is well-defined if its members can be definitely identified. “Tall students”, “good books” and “beautiful flowers” depend on personal opinion and are not well-defined. The set of prime numbers less than 20 is well-defined.

Hence, option 3 is the correct option.