We have not decided, but we're thinking ___ buying a new car.

1. at
2. of
3. about
4. on

Both “think of” and “think about” can be used here, with slight nuance differences. “Thinking about” often implies more detailed consideration or contemplation, while “think of” might suggest a passing idea. In modern usage, both are acceptable for considering options. The question accepts “about” as correct, though “of” would also work. Understanding these nuances helps express thought processes accurately.