Final answer:
The correct form of grande to match the feminine plural noun mochilas is grandes, as Spanish adjectives agree with the gender and number of the nouns they modify.
Step-by-step explanation:
The correct form of grande to use with mochilas (which is a feminine plural noun in Spanish, meaning backpacks) is grandes. In Spanish, adjectives must agree in gender and number with the nouns they modify. Since mochilas is feminine plural, grande changes to grandes to match. This is a rule that applies generally in Spanish grammar.