The word جَدَّتك is composed of جَدَّة = grandmother and the suffix ـك which indicate possession of the 2nd pers. sing. (i.e. your). Thus, جَدَّتك = your grandmother
N.B.: There is a mistake in the exercise. It's supposed to be جَدَّتُك (pronounced /d͡ʒə.'dːæ.tʊ.ka/), since your grandmother is the subject of the sentence.