Which words in the sentence are the complete infinitive phrase?
dad only wants to serve the veggie burgers.
a.
dad only wants
b.
only wants to serve
c.
to serve the veggie burgers
d.
to serve

D - to serve. This is because the infinitive verb is the form of the verb before it has been conjugated to gain a person (I, you, he/she/it, we, you plural, they) and a tense. In English, such verbs are expressed as 'to ', and this is how you will see them when you look them up in the dictionary.

I hope this helps