the state in which opposing forces are balanced
Though each of the answers may appear to be correct, the description of "opposing forces" can easily give a hint to readers about the validity of this answer. Equilibrium
concerns a state of balance, though that balance may involve parts of the other answer choices.
B the principle describing the notion that every action must have a reaction
C the presence of motion in the present day which serves as a means of understanding movement as a whole
D the state in which similar forces attract, causing a swell of movement in a single direction