I don't know how to do this task, please help me.

Question

The sequence of Latin letters is constructed as follows: first it is empty. At each successive step, the sequence doubles, after which the regular letter of the Latin alphabet is appended to it (A, B, C,...). Define a character that stands on nth position in a sequence received after a step that is set by a user

Answer 1

My understanding of your task description, it builds up the sequence like this
A
AAB
AABAABC
AABAABCAABAABCD
and so on.
You can use a list to build up the elements.
List.Add() to append a letter to the end.
List.AddRange() to append the whole list to itself.
Repeat this until the size of the list is at least n, then take the nth element.

Edit: actually you can do the same without lists, just using string...