Final answer:
Option b) Melting snow turned the mountain's quiet streams into raging rivers is the best fit for the statement, since the unusual warmth would cause snow to melt and streams to swell.
Step-by-step explanation:
The best fit for the empty box preceding 'The weather was unusually warm on Mount Washington' is b) Melting snow turned the mountain's quiet streams into raging rivers.
This is because the unusually warm weather would cause the snow to melt faster than usual, leading to a significant increase in water flow in streams, turning them into raging rivers. This option creates a logical cause and effect relationship based on the given weather condition.