Which of the following is an effect of the electric force?

a. a positive and negative charge attract each other.
b. a positive and negative charge repel each other.
c. a north pole and south pole attract each other.
d. a north pole and south pole repel each other.

As we know that two charges exert force on each other when they are placed near to each other

The force between two charges is given as

here we know that

= two different point charges

r = distance between two point charges

also we know that two similar charges always repel each other while two opposite charges always attract each other

so here correct answer would be

A. A positive and negative charge attract each other.