The correct answer is: A dangling modifier.
Modifiers help us add more detail about our thoughts and sentences. "She spoke quickly when she saw her mother". Here, the word quickly is a modifier, modifying the word spoke. It goves more information about how someone spoke (an adverb modifying a verb).
A dangling modifier is usually placed at the beginning of a sentence, although we can also put it at the end of a sentence.
Walking on the road, a plane flew over the sky. In this sentence, we don't know who was walking on the road, so the modifier walking on the road doesn't have anything to modify.
