What is the facility that allows nesting one select statement into another?

Answer: Option C

Imagine you have a big box filled with smaller boxes. You want to find a specific item, but you need to open the bigger box first to see what smaller boxes are inside. Then, you need to open those smaller boxes to find your item.

This is similar to how subquerying works in MySQL. It allows you to run one query (like opening the big box) inside another query (like opening the smaller boxes) to get specific information.

So the answer to the question is Option C: subquerying.