Question

two balls of the same size are moving at the same speed when they hit a wall.
ball a is made from soft material,
and ball b is made from hard material.

what is true about the forces
exerted onto each ball?
the force exerted onto both balls is the same.
the force exerted onto ball a (soft) is smaller.
the force exerted onto ball b (hard) is smaller.

Explanation:

This relies on the impulse-momentum theorem ($F\Delta t = \Delta p$). Both balls have the same mass and initial speed, so they have the same change in momentum $\Delta p$ when stopping (or bouncing, but the key is $\Delta p$ is equal for both). The soft ball (A) deforms, increasing the time $\Delta t$ of impact. Since $F = \frac{\Delta p}{\Delta t}$, a larger $\Delta t$ leads to a smaller impact force on ball A.

Answer:

The force exerted onto ball A (soft) is smaller.