Explanation:

Let the two digit number be 'ab' and the number formed after adding 18 to it is 'ba'
So ab + 18 = ba
⇒ (10a + b) + 18 = 10b + a
⇒ 18 = 9b – 9a
⇒ 2 = b – a
So we know that ab, ba both are two digit numbers so a, b ≠ 0.
Also when b = 9, a = 7; b = 8, a = 6; b = 7, a = 5; b = 6, a = 4; b = 5, a = 3; b = 4, a = 2; b = 3, a = 1
Total 7 combinations are possible.

Hence, option 3 is the correct answer.