Final answer:
To determine when monarch butterflies covered the greatest area in Mexico during winter, one needs specific migratory data, which is not provided in the information available.
Step-by-step explanation:
The question seeks to find out during which winter season monarch butterflies covered the greatest total area at their overwintering sites in Mexico. This question pertains to the migratory behavior of monarch butterflies, which involves them flying up to 3,000 miles from their summer habitats in North America to their winter habitats in Mexico. This phenomenon is an example of one of the longest known migrations in the insect kingdom.
To provide an accurate answer, one would need to reference specific data from a reliable source that records the yearly coverage of monarch butterflies at their wintering grounds. Unfortunately, the provided information does not include data for the specific winter seasons in question.