A word which is the possessive pronoun being used as an adjective in the following sentence is "her."
The possessive pronoun refers to the word which is used in place of a noun. For example, yours, hers, theirs etc
Whereas personal adjectives refer to the words which describe a noun. For example, in your book, my sister, his car etc.
Therefore in the above sentence, "Amanda went shopping after she ate her lunch." Her lunch is used as an adjective, as lunch as noun is described to belong to Amanda.
