__________ ambiguity occurs when a phrase or sentence has more than one underlying structure, such as the phrases.

The type of ambiguity you are referring to is called structural ambiguity (or syntactic ambiguity). Structural ambiguity occurs when a phrase or sentence can be interpreted in multiple ways due to its grammatical structure. This can lead to different meanings based on how the words are grouped or related to one another, often depending on the arrangement of phrases within a sentence.

For example, the sentence "The old man and woman sat on the bench" could be interpreted as either referring to one old man and one old woman or as two old individuals (a man and a woman) who are old. Each interpretation stems from the different possible structures that can be inferred from the words used.

Understanding structural ambiguity is vital in linguistics, poetry, and other forms of communication where nuance and interpretation play significant roles in meaning.