Question

A man standing on a bus remains still when the bus is at rest. When the bus moves forward and then slows down the man continues moving forward at the original speed. This is an example of the effect of
A. gravity.
B. weight.
D. inertia.

Answer 1

When the bus starts moving forward, the man stays still, so
he leans back.  When the bus slows down, the man continues
moving forward at constant speed, so he leans forward. 
All of this is a demonstration of the effects of inertia.

Answer 2

D. Inertia 
Trust me, I know.