Step 1: Concept
Prepositions of time used for long periods such as months, years, and seasons.
Step 2: Grammar Rule
- In is used for months (In December), seasons, and years.
- On is used for specific days and dates (On Monday).
- At is used for precise times (At 5:00).
Step 3: Conclusion
Since December is a month, the correct preposition is "in."
Final Answer: (4)