* a. mass and velocity

Here's why:

* Momentum is a measure of an object's motion. It's a combination of how much "stuff" an object has (mass) and how fast it's moving (velocity).

* Force is a push or pull that can change an object's momentum.

* Acceleration is the rate of change of velocity. While it's related to momentum, it's not a direct determinant.

* Gravity is a force that affects an object's motion and can therefore change its momentum, but it's not a factor that directly defines momentum itself.

The formula for momentum is:

Momentum (p) = mass (m) x velocity (v)