The highest clouds are:

Correct option is A

Cirrus clouds are the highest clouds in the Earth's atmosphere.
They form at altitudes ranging from 6,000 to 13,000 meters (20,000 to 43,000 feet), typically in the upper troposphere.
These clouds appear thin, wispy, and feather-like and are composed of ice crystals due to the very low temperatures at high altitudes.
Here's a quick comparison of cloud types based on altitude:
(a) Cirrus: ✅ Highest altitude, above 6 km. Made of ice crystals.
(b) Stratus: Low-level clouds, often forming at below 2 km.
(c) Cumulus: Low to middle-level clouds, usually found around 2–6 km.
(d) Cumulo-nimbus: Vertical clouds that span low to high altitudes, but their base is low, and only their tops reach higher levels.