Question

what must be happening, according to newtons second law of motion, if a rigid body of unchanging mass is observed accelerating?

a force was once applied but is now not being applied.
the mass must be changing.
no force was or is being applied to the mass.
a force is being applied to the mass.

Explanation:

Newton's second - law is $F = ma$, where $F$ is force, $m$ is mass and $a$ is acceleration. If a body of unchanging mass ($m$ is constant) is accelerating ($a
eq0$), then from the formula $F = ma$, a non - zero force $F$ must be acting on it.

Answer:

A force is being applied to the mass.