To calculate the force one person exerts on the other during a collision, you primarily need to determine the change in momentum per unit time, which can be related to the accelerations involved.
Given this, the correct answer is:
a. the mass and acceleration of each person
Knowing the mass allows you to calculate the momentum, and the acceleration (which would be the change in velocity over time) is needed to determine the force using Newton's second law (F = ma). While speed and velocity provide information about how fast they were moving before the collision, it is the mass and acceleration that directly relate to the force exerted during the impact.