Good idea – I wish I had thought ___ it.

1. at
2. of
3. under
4. for

“Thought of” is the correct preposition combination when referring to having an idea or consideration. The phrase “I wish I had thought of it” expresses regret about not having previously considered something. “Think of” introduces ideas, while “think about” involves contemplation. This distinction helps learners express idea generation and regret accurately in conversation and writing.