Final answer:
In the summer when the wind is calm and the sky is clear, the minimum temperature is typically reached during the early morning, just before sunrise.
Step-by-step explanation:
In the summer when the wind is calm and the sky is clear, the minimum temperature is typically reached during the early morning, just before sunrise.
This is because after sunset, the ground and air begin to cool, and without the Sun's warmth during the night, the temperature continues to drop until just before sunrise, when it is usually the coldest.
Therefore, the correct answer is a. Just before sunrise.