Which of these is a repulsive force?

Think it through first, then pick an option.

Answer and explanation

Correct answer: a Electromagnetic force

Electric and magnetic forces both attract and repel; a magnet can pull another magnet and also push it away.

Why the other options are wrong

b Gravitational force: Gravity only attracts; objects with mass pull one another towards themselves.
c Weak nuclear force: The weak nuclear force shows up as beta emission; the book does not call it a repulsive force.
d Strong nuclear force: The strong nuclear force holds nuclear particles; the book does not call it a repulsive force.