Final answer:
"The past behind her" is an adverbial phrase, typically providing details about time or place in relation to the verb.
Step-by-step explanation:
"The past behind her" is a phrase that functions within a sentence to provide additional details. Based on the information provided, it's likely that this phrase is serving a specific grammatical purpose. Prepositional phrases often take the form of adverbials, which are used to modify verbs, adjectives, or other adverbs, typically providing information about time, place, manner, or reason. Considering the example provided and standard sentence patterns that occur with the be verb, when a phrase follows a form of 'be', it often acts as an adverbial of time or place. This makes option (a) Adverbial phrase the most appropriate choice for "The past behind her."