Correct Option (D)
The passage states that polar bear mothers emerge from their dens in spring with three-month-old cubs. If the cubs are three months old in spring, their birth must have occurred approximately three months prior to the onset of spring. This timeline places the birthing period within the winter season (typically December to January).
Incorrect Options:
Spring: The text indicates that spring is the season when mothers emerge with already three-month-old cubs, not when the births occur.
Summer and Autumn: These seasons follow spring. If births occurred in summer or autumn, the cubs would be considerably older than three months by the time of their emergence in spring, which contradicts the information provided in the passage.