Answer:
Carlos las tiene.
Ellos van a comprarlos.
Tú las sacas.
Nosotros la hacemos.
Mario y Gabriela están buscándolo.
El no los está esperando.
Explanation:
The word that is replacing the noun needs to match the gender of the word it is replacing. Keys or llaves are feminine, so you use las not los. Also, the word needs to match the quantity. Llaves is plural so we use las, not la.
Atender, working on the same thing, its hard
*Así* is the correct way to write this word. *Hací* Is a barbarism and, therefore, it is convenient to avoid it.