Select the letter that can replace the question mark (?) in the following series.

A, E, I, ?, Q, U

The series given is: A, E, I, ?, Q, U

Let's find the position of each letter in the alphabet:

A = 1st position

E = 5th position

I = 9th position

Q = 17th position

U = 21st position

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Finding the pattern by examining the differences between consecutive positions:

E - A = 5 - 1 = 4

I - E = 9 - 5 = 4

Q - ? = difference should be 4

U - Q = 21 - 17 = 4

The pattern shows each letter is 4 positions ahead of the previous letter.

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To find the missing letter:

I is at position 9

The next letter should be at position: 9 + 4 = 13

The 13th letter of the alphabet is M.

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Verification of complete series:

A (1), E (5), I (9), M (13), Q (17), U (21)

All consecutive differences equal 4.

Therefore, the answer is Option 2: M