Final answer:
Lenticular clouds are the best visual indicator of a mountain wave among the three types of clouds.
Step-by-step explanation:
The best visual indicator of a mountain wave among the three types of clouds is lenticular clouds (option c). Lenticular clouds form when moist air flows over a mountain or other elevated terrain, creating a wave-like pattern in the atmosphere. The air cools and moisture condenses in the crest of each wave, forming these distinct lens-shaped clouds.
Unlike cumulus clouds (option a) which are puffy and often indicate fair weather, lenticular clouds are stationary and form only under specific conditions involving strong winds and stable air. Cirrus clouds (option b) are high-altitude clouds that consist of ice crystals, and they do not indicate mountain waves. Stratus clouds (option d) are low-level clouds that form in a uniform layer and are not associated with mountain waves.