Answer:
The function of the gerund phrase in the sentence is:
C. subject.
Step-by-step explanation:
A gerund phrase is a group of words which includes a gerund plus any complements or modifiers. It functions as a noun in the sentence.
In the sentence we are analyzing here, the gerund phrase is "riding a roller coaster." To determine its function, we can ask the verb a simple question: What scares my little brother? Riding a roller coaster. As we can see, the gerund phrase is the subject of the sentence, the topic to which the verb refers. Therefore, letter C is the best answer for this question.