[susan53]

1 = c : want to

With verbs followed by to + infinitive, you can always use the verb + to as an ellipted version of a complete verb phrase, where the rest of the verb phrase is retrievable from the context. Some more examples :

1. He knows he can't go on the trip, but he'd like to.
2. A : Isn't it about time you started studying for the exam? b: Yes, I know I need to.
3. A : Have you ever seen the Northern Lights? B : No, but I hope to when I go to Finland this winter.

It's obvious from the context that these ellipted forms mean he'd like to go on the trip / I need to start studying/ I hope to see the Northern Lights .

The same thing is happening in your example. because it's a verb + infinitive construction, you have the choice between using an ordinary short form answer using the operator :

A . Do they want to reserve a room? B : Yes, they do.

which would probably be more usual here, or the verb + to construction - which would be more likely to be used if the speaker went on to qualify what s/he had said :

A : Do they want to reserve a room? B : Well yes, they want to, but they're not sure of the dates.

As the example stands, the verb + to, though grammatically correct, sounds unnatural. It's not a very good question.

You'll find another example in the answer to the next question if you look for it!