1. La estudiante los compra.
2. Él lo factura antes de abordar.
3. Mi mamá lo necesita.
4. Yo las hago antes de viajar.
5. La agente de viajes lo hace.
6. Yo lo pongo en la maleta.
7. Los estudiantes no la hacen.
Explanation:
It is called a direct object (in traditional grammar, direct complement) to the function played by a syntactic constituent (phrase, pronoun or a substantive subordinate proposition), different from the grammatical subject, which is necessarily required by a transitive verb
it is the person or object on which the action expressed by the verb falls, it is a modifier of the predicate. The direct object can be expressed by pronouns: lo, la, los, las. Substituting the direct object of a sentence for one of these pronouns is a recommended way to verify that it is indeed the direct object.