Answer:
C. rules
Step-by-step explanation:
Modifiers give more details about the modified word. They can be in a form of a single word or more complex phrases or dependent sentences.
For example:
Book
Blue book
A blue book laying on the table.
A very well written blue book that is laying on the table.
We used all these words and phrases just to modify the noun "book" and provide more information about it.
In the given example, the phrase "incredibly simple to understand and use" gives us more details on "rules", which means it modifies that word.