Will
in which situation is the object experiencing unbalanced forces?
a) a box resting on a horizontal floor.
b) a car slowing as it reaches a traffic light.
c) a car with its cruise control set to 30 km/h.
d) a penny at terminal velocity after falling off of a building.

A car slowing as it reaches a traffic light, because it is the only option with acceleration/ deceleration.
And forces require the mass of the object AND the acceleration.

An  atom  (ours, not carroll's) consists of a positively charged  nucleus, surrounded by one or more negatively charged particles called  electrons. the positive charges equal the negative charges, so the atom has no overall charge; it is electrically neutral. most of an atom's mass is in its nucleus; the mass of an electron is only 1/1836 the mass of the lightest nucleus, that of hydrogen. although the nucleus is heavy, it is quite small compared with the overall size of an atom. the radius of a typical atom is around 0.1 to 0.25 nanometres (nm), whereas the radius of a nucleus is about 10-6  nm.  *  if an atom were enlarged to the size of the earth, its nucleus would be only 200 feet in diameter and could easily rest inside a small football stadium.read more on -