Right answer: ar verbs
The infinitive is one of the non-personal forms of the verb.
In Spanish grammar the infinitive has two forms:
1) Simple Infinitive: has three possible endings: -ar, -er, -ir
2) Compound Infinitive: it is formed with the verb "haber" (to have) + the participle of the verb we want to conjugate
In this case we are talking about the first form (Simple Infinitive), especifically the ar verbs, because cantar (to sing), bailar (to dance), caminar (to walk), amar (to love) end in ar.