Explanation:

Correct option:2

Solution: The given set={15,21,27}

The pattern followed here is: {15, 21, 27} → 21 - 15 = 6 and 27 - 21 = 6 means equal gap between consecutive numbers.
Similarly, {20, 28, 36} → 28 - 20 =8 and 36-28=8 

Hence {20, 28, 36} is the set of numbers that is similar to the following set of numbers. {15, 21, 27}