Based on the student's observations over the days, the sequence is as follows:
- Day 1: sunny and warm
- Day 2: cloudy and rainy (likely indicating the arrival of moisture)
- Day 3: drop in temperature (this suggests a change in air mass, typically associated with a cold front)
- Day 4: clear skies and cold temperatures (indicating that after the cold front passed, the weather has cleared up but remained cold)
Considering these observations, the most fitting explanation for the observed weather patterns is that a cold front moved into the area.
So, the correct response would be: A cold front moved into the area.