Answer:
The combinations are
1.
C , 
2.
C , 
Explanation:
The force between two charges q and Q is given by Coulomb's law.
The coulomb force is given by

Where, r be the distance between two charges
According to the question,
r = 15 cm, Q =
C, q =
C , F = 12 N
Force remains same, distance remains same
so the product of Q and q be same
The product of Q and q is 
So other combination of charges are
1.
C , 
2.
C , 