If the first and the third letters in the letter group DISTRIBUTION are interchanged and also the second and fourth letters, the fifth and seventh and so on then which of the following would be seventh letter from the left?

R

Following is the sequence in which letters are interchanged:

1 → 3, 2 → 4, 5 → 7, 6 → 8, 9 → 11, 10 → 12

Before change: 

After change: 

7th letter from the left is  "R"