Final answer:
Sentence (4) provides evidence that best supports the main idea of the paragraph by explaining the necessary conditions for glacier formation.
Step-by-step explanation:
The sentence that provides evidence that best supports the main idea of the paragraph is sentence (4): 'In order for glaciers to form, the climate must be cold and snowy in the winter and fairly cool in the summer.' This sentence explains the necessary conditions for the formation of glaciers, emphasizing the importance of cold temperatures and snowfall in the winter. The sentence supports the main idea that glaciers are created through the compaction of layers of snow and ice.