Question

need full explanation

Answer 1

for attraction charges should be of opposite nature
so its product would be negative or less than zero

Answer 2

If q1q2>0, both the charges must be positive. Also, like charges repel each other. Hence, the force will be repulsive.

If q1q2<0, one of the charges must be negative and other must be positive. Also unlike charges attract each other. Hence, the force will be attractive.
Hope this helps!