In the series of letters provided (d, f, h, j, k, n, p, r), the letter that does not belong is k.
To understand why, let’s look for patterns in the series. If we analyze how the letters are spaced, we can see that:
- d to f: +2 positions
- f to h: +2 positions
- h to j: +2 positions
- j to k: +1 position
- k to n: +3 positions
- n to p: +2 positions
- p to r: +2 positions
Every letter progresses in a sequence with the majority experiencing an increment of +2, except for the jump from j to k, which is only +1. This inconsistency suggests that k is the odd one out in this series.