Finding the letter at a specific position in an expanding series pattern

Identify the structural groups in the string: Group 1: A (length 1) Group 2: A, B (length 2) Group 3: A, B, C (length 3)

The total number of letters up to the end of the n-th group is the sum of the first n natural numbers: S_n = n(n+1)/2.

Find the largest group that finishes just before the 100th position: If n = 13, S₍13₎ = 13 × 14/2 = 91. This means the 13th group ends exactly at the 91st letter of the sequence.

The 14th group begins at the 92nd letter and resets the alphabet: A, B, C, D... We need the 100th letter, which is the 100 - 91 = 9th letter of this 14th group.

The 9th letter of the English alphabet is I.

Answer: (c).