If the total momentum of a system is changing:

(a) particles of the system must be exerting forces on each other
(b) the system must be under the influence of gravity
(c) the center of mass must have a constant velocity
(d) a net external force must be acting on the system
(e) none of the above

(d) a net external force must be acting on the system

Explanation:

Momentum is given as the product of mass and velocity.

P = MV

According to Newton's second law of motion, " Force applied to a body (system) is directly proportional to the rate of change of momentum of the body (system) which takes place in the direction of the applied force (external force).

F ∝ΔMV

Therefore, If the total momentum of a system is changing, a net external force must be acting on the system.

(d) a net external force must be acting on the system

the electric field is a “force field” around a charged object that illustrates the direction the electric force would push an imaginary 

0.11 m/s and 0.28 m/s should be correct