The students found the grammatical rules incredibly simple to understand and use.
which word does the underlined phrase modify?

The word that the underlined phrase "grammatical rules" modify is rules.

Explanation:

Taking in consideration de phrase "incredibly simple", the direct noun that is modified by this adverb and adjective is rules, since they describe how the rules seem to be to the students, "The rules are incredibly simple to understand", "incredibly" modifies "easy" and then the two words together are connected to "rules."

assuming the underlined word is "regnant", the correct answer is b) ruling.

dependent clause!

example: "before the food gets cold"