Final answer:
The correct word is 'généreuse' to match the gender and number of the noun.
Step-by-step explanation:
The correct word to fill in the blank is généreuse.
In French, adjectives agree in gender and number with the noun they modify. Since grand-mère (grandmother) is a singular feminine noun, the adjective must also be singular feminine. The correct form of the adjective is généreuse to match with the feminine noun.
For example, you could say: Sa grand-mère est très généreuse. (Her grandmother is very generous.)
Learn more about Adjectives in French