Final answer:
The subject of this question is Mathematics and the correct answer is option D) Recursive Odd-Length Strings.
Step-by-step explanation:
The subject of this question is Mathematics and the grade level is High School.
The correct answer to the question is option D) Recursive Odd-Length Strings.
Recursive Definition is a method of defining objects or structures in terms of themselves. In this case, the question is asking for a recursive definition where all the strings have odd lengths. Option D correctly captures this by describing the strings as recursive and of odd lengths.