What location do Nick and Marta tell their grandmother they are going to?

In the scenario provided, the correct answer is that Nick and Marta tell their grandmother they are going to the mall to watch a movie. This choice reflects a common social activity for young people who often enjoy going to the mall for entertainment purposes, such as watching the latest films.

The idea of visiting the mall for a movie suggests a casual outing that aligns well with typical patterns of leisure among friends. It often involves a combination of shopping, dining, and socializing, making it a relatable choice for many individuals in that age group.

Other options suggest activities with different contexts. Studying at the library leans more towards academic responsibilities, while going to the park for a picnic denotes an outdoor activity that may not necessarily involve the location's primary purpose for entertainment. Lastly, visiting a friend’s house for a gathering indicates a private social setting rather than a public one associated with a film viewing. Thus, the choice involving the mall not only resonates with typical behavior but also sets a scene of a fun outing, making it the most fitting option.