A compact car and a large truck collide head on and stick together. Which undergoes the larger change in the magnitude of momentum?

Neither. They will have equal change in the magnitude of momentum.

Explanation:

Change in in momentum of a body is calculated as follows:

ΔP = FΔt

where ΔP is the change in momentum

and F is the force exerted on the object

and Δt is the change in time during the impact

The time it takes for the impact is the same for both objects. Also, according to Newton's third law, when object A exerts a force on on object B, B exerts an equal (magnitude wise) but opposite force on A. Therefore the value of F is also equal. This tells us that the magnitude of impulse will be the same for both the fly and the truck