Draw the electric field lines around a system that consists of two equal negative charges.

The momentum of each cart is given by:

where
m is the mass of the cart
v is its velocity (at the bottom of the ramp)

To answer the problem, let's calculate the momentum of each of the 4 carts:
1)
2) 
3) 
4) 

Therefore, the cart with greatest momentum is cart 3, so the right answer is
- trial 3, because this trial has a large mass and a large velocity