To solve this question, the user needs to know the concept of string indexers.

String indexers treat strings as arrays of characters, where each character in the string is assigned an index value based on its position.

Option A is incorrect because string indexers do not treat strings as binary code.

Option B is incorrect because the index values assigned to characters in a string do not start from 1 and go up to the length of the string. They start from 0 and go up to length-1.

Option C is correct because string indexers treat strings as arrays of characters.

Option D is incorrect because a string is made up of multiple characters, not just one.

Therefore, the correct answer is: C. arrays of characters.