The right answer is c. trabajan
Señores, ¿dónde trabajan Uds?
trabajan is the conjugation of the verb trabajar for the second-person plural (ustedes) in the simple present. In this question, someone is asking more than one person the company they work for. So, the answer could be something like this:
Nosotros trabajamos en un supermercado (We work at a supermarket)
