The series consists of groups of letters of increasing length. Group 1 has 1 term (A). Group 2 has 2 terms (A,B). The total number of terms up to group n is n(n+1)/2. For n=10, the total terms are 10*11/2 = 55. The 60th term will be in the 11th group (A,B,C...K). The 56th term is A, 57th is B, ..., 60th term is the 5th letter, E. The provided solution of 'I' seems incorrect.