Explanation:

To find out which letter series follows the given rule, we need to analyze each option and determine if the number of letters skipped in between adjacent letters of the series starting from behind increases by one.

Let's analyze each option:

a) ONLKJ
The letters in this series are O, N, L, K, and J. If we count the number of letters skipped in between adjacent letters starting from behind, we have:
O - N: 0 letters skipped
N - L: 1 letter skipped
L - K: 2 letters skipped
K - J: 3 letters skipped

Based on the given rule, the number of letters skipped should increase by one, but in this series, the number of letters skipped decreases. Therefore, option 'a' does not follow the given rule.

b) OMKIG
The letters in this series are O, M, K, I, and G. If we count the number of letters skipped in between adjacent letters starting from behind, we have:
O - M: 0 letters skipped
M - K: 1 letter skipped
K - I: 2 letters skipped
I - G: 3 letters skipped

Based on the given rule, the number of letters skipped should increase by one, and in this series, the number of letters skipped does increase. Therefore, option 'b' follows the given rule.

c) OIGDC
The letters in this series are O, I, G, D, and C. If we count the number of letters skipped in between adjacent letters starting from behind, we have:
O - I: 0 letters skipped
I - G: 1 letter skipped
G - D: 2 letters skipped
D - C: 3 letters skipped

Based on the given rule, the number of letters skipped should increase by one, and in this series, the number of letters skipped does increase. Therefore, option 'c' follows the given rule.

d) OMJFA
The letters in this series are O, M, J, F, and A. If we count the number of letters skipped in between adjacent letters starting from behind, we have:
O - M: 0 letters skipped
M - J: 1 letter skipped
J - F: 2 letters skipped
F - A: 3 letters skipped

Based on the given rule, the number of letters skipped should increase by one, and in this series, the number of letters skipped does increase. Therefore, option 'd' follows the given rule.

Conclusion:
Option 'D' (OMJFA) is the letter series that follows the given rule, where the number of letters skipped in between adjacent letters of the series starting from behind increases by one.