Hi

This is easy, the answer is GHI

EXPLANATION

Let us number the each group of alphabets

LMN,    ABC,  OPQ,   DEF,  RST,   ______

1           2        3         4         5         6

You can observe that there is continuation of Alphabets in their normal order at alternate places

i. e we have LMN at 1st place, OPQ comes after LMN in the normal order which is nothing but your 3rd place. Similarly, we have ABC at 2nd place and DEF at 4th place , again we have OPQ at 3rd place and RST at 5th place.

Now,

We have to find alphabets at 6th place .

According to above pattern, Alphabets at 6th place will be

= next three alphabets of alphabets at alternate place of 6th i. e 4th place

= next three alphabets after DEF ( as DEF is at 4th place)

= GHI

So, 6th place /missing alphabets = GHI

Hence, the complete series is :-

LMN, ABC, OPQ, DEF, RST, GHI

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