Here's link to the answer
The books? not really sure what singular means but estos libros means the books'
1. I'm 14 years old. And Maria?
2. You're 12 years old. And the boys?
3. Mom is 40 years old. And dad?
4. You're 15 years old. And our friend?
The right answer is b) el dinero, it’s the direct object of the phrase “Give me the money now, please.”