Two dump trucks hauling rock to a construction site is given by the expression. xP + yQ 
P + Q 
Truck 1 can haul x cubic yards 
Truck 2 can haul y cubic yards. 
Truck 1 makes P trips to the construction site, while truck 2 makes Q trips. 
Hence the trips made by truck 1 and truck 2 should be the addition of the trips made by the trucks individually.
So the total trips is represented by P + Q
Option C) is the right answer