Where are Nick and Marta heading when they spot a car identical to Mrs. Starch's?

The correct answer reflects a scenario in which Nick and Marta see a car that closely resembles Mrs. Starch's car while they are at an appropriate location for such an observation. In many narratives where characters are on a journey or encountering significant events, a Circle K, as a convenience store, could be a common stop for young characters in search of snacks, drinks, or a quick errand. This setting adds a realistic aspect to the story and primes interactions or developments that might occur at such a place. The mention of a car identical to Mrs. Starch's may imply some connection to their current activities or narrative relevance at that location.

The other options, while plausible locations, don't align as closely with the typical context where the characters might notice something as specific as a car in passing. For instance, a grocery store suggests a more extended shopping purpose, the park is usually a leisure destination with different focal points, and school is primarily associated with academic activities rather than casual encounters with vehicles. Therefore, being at Circle K enhances the narrative by allowing an unexpected plot twist or moment of recognition regarding Mrs. Starch's identical car.