Las visitas registrar sus nombres en la recepción para entrar al edificio. es necesario necesitan hay que deber

We must fill in the blank with a verb conjugated in simple present that tells us that the visitors must or need to register their names at the reception to enter the building. From the option, the only word that meets the requirements is necesitan that is the conjugation of the verb necesitar for the third person plural in the simple present. Finally:

Las visitas necesitan registrar sus nombres en la recepción para entrar al edificio.

well, if he says unos it is 1, if he says dos, it is 2, if he says tres, it is 3, if he says cuatro, it is 4

c. its asking which of the words need an accent mark. its spelled biografía