This one

Lesson 21

Which car do you like? This one.
Which socks do you want? These ones.

Which pen do you want? The blue one.
Which apples did you buy? The red ones.

When we answer questions, we can use one or ones instead of the noun.

This is our mouse. He is the only one.
The sixth planet was ten times bigger than the last one.

You can see more examples in the book The Little Prince.