Here's a beginner friendly explanation!
This is a easy level question and the ideal time to be taken in the GMAT is 30-45 seconds.
The average of any odd number of consecutive integers is the middle number. If it is 1, 2, 3, 4, 5; the average is 3(the middle number). If it is 12673, 12674, 12675, 12676, 12677; the average is 12675(the middle number; trust me, I calculated it :p)
Approach: jot down how can the 5 consecutive integers that starts with m can be. In most questions, jotting it down will give you an idea on how the question is set.
The 5 numbers can be m, m + 1, m + 2, m + 3, m + 4. As explained earlier, for the consecutive odd integers, the average is the middle term i.e., m + 2.
It is given that the average is n. So, m + 2 = n.
The 9 consecutive numbers that start with m + 2 will be m + 2, m + 3, m + 4, m + 5, m + 6, m + 7, m + 8, m + 9, m + 10. The average is the middle term, i.e., m + 6. There isn't an option that matches with our answer. If m + 2 = n as we derived in the previous step, m + 6 will be n + 4(adding 4 on both sides to get the required new average).
Option E matches with n + 4.