Answer is option (c)
Since a and b are positive integers, let's take some particular value for a and b and try to solve it.
Say a=23 and b=5. So a/b= 23/5= 4 and a%b= 23%5 =3
Option 1. 4*5+3=23 != b(as b is taken to be 5). So its wrong.
Option 2. 3*5+4=19 !=b. So its wrong
Option 3. 4*5+3=23= a. Its correct.
Option 4. 3*5+4=19 !=a. So its wrong.
Correct me if I am wrong.