Force balance in electric field Steps:

• The oil drop is stationary, so gravitational force mg equals electric force neE.
• Electric field E between plates is V/d.
• Substitute: ne(V/d) = mg.
• Solve for n: n = (mgd)/(eV).

Why B is correct:

• Not applicable; no option matches the derived formula.

Why the others are wrong:

• A omits d, assuming E = V instead of V/d.
• C rearranges terms incorrectly, yielding inverse dimensions.
• D inverts the formula, giving 1/n with swapped factors.

Not enough information to select a matching option; derived n = (m g d)/(e V) not present.

Final answer: Not enough information.