The phrase that is underlined (Grandma Mary) is an example of an "appositive phrase".
The reason why it's (A) is because an appositive phrase basically restates the noun after it in a different way. Grandma Mary is a noun (remember, person/place/thing).
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The reason why the other two options are incorrect is because an adjective phrase is because an adjective is not described the noun.
It's definitely not an adverb phrase because it's not modifying an adverb with the words, "how/when/why?"
