You can solve this by rough estimation given you have multiple choice here. You can quickly see that each of the first 3 answers satisfy the condition that the father is currently 5 times the age of the son.
By subtracting 4 years from each age in each answer you will see them as follows:
a: 26 and 2
b: 21 and 1
c: 36 and 4
Now it is clear that the answer is c: 40 and 8 (because 4 x 9 = 36 and 5 x 8 = 40). These questions are why it is important to learn your multiplication tables prior to testing.