What should come next in the following letter series ?
ACEGIKMOBDFHJLNACEGIKMBDFHJLA

This series is made from a combination of two series from the first 15 alphabets of the English alphabet.

First series is : The letters at odd positions => A C E G I K M O

Second series is : The letters at even positions => B D F H J L N

Then, the last letter from each series is removed. Thus making it : A C E G I K M B D F H J L

Again, after removing the last letter(i.e. M & L) => A C E G I K B D F H J

Clearly, the next letter after A is 'C'.