Look at the indirect-object: a Uds. That is plural so you need a plural indirect-object pronoun. After Qué (what) it is natural to say "is" which is sin gular.
b. les falta (literally the translation is "what is lacking to you-all?" but it is translated "What do you-all need?"
Sra
¿Qué _____ a Uds.?
a. le falta
b. les falta
c. le faltan
d. nos falta
A?
2 answers
¡Gracias! I wasn't sure if it was singular or plural.