Her chorus director does not appreciate Angela's fidgeting.
A gerund is a verb ending in /ing/ that is used as a noun. In this sentence "fidgeting" is a noun because it's something Angela can own. Also, if you replaced "fidgeting" with another noun like dog, it would still work perfectly. In the first option "fishing" is used as a verb. In the second sentence "joining" is used as a verb. In the third sentence "dangling" is used as an adjective to describe the string. String is not a verb acting as a noun in this sentence. It is simply a noun since it is a piece of fabric twisted tightly.
