Clouds are the masses of small water droplets or tiny ice crystals.

(A) Clouds are classified according to their appearance and height.

(B) Cirrus clouds are high altitude clouds, which are usually feathery shaped and composed entirely of ice crystals.

(C) Nimbostratus clouds are mid level clouds producing sporadic rain.

(D) Altocumulus are the heap-like clouds having flat bases and rounded tops.

Choose the correct answer from the options given below: 

The correct answer is (A), (B) and (D) only.

Let us analyze each statement step-by-step to understand why this option is correct:

Statement (A): Clouds are classified according to their appearance and height.
This statement is correct. Globally, clouds are classified into categories based on their altitude or height (high-level, mid-level, and low-level clouds) and their physical form or appearance (such as fibrous/wispy, sheet-like/layered, or heap-like/puffy).

Statement (B): Cirrus clouds are high altitude clouds, which are usually feathery shaped and composed entirely of ice crystals.
This statement is correct. Cirrus clouds typically form at high altitudes (generally above 6,000 meters or 20,000 feet). Because temperatures at these heights are extremely low, they are composed almost entirely of ice crystals, giving them a thin, wispy, and feathery appearance.

Statement (C): Nimbostratus clouds are mid level clouds producing sporadic rain.
This statement is incorrect. Nimbostratus clouds are typically classified as low-level clouds (though they can extend into middle levels) and are characterized by a thick, dark, and gray layered appearance. Crucially, they are known for producing continuous, steady, and widespread precipitation (rain or snow), rather than sporadic or brief showers.

Statement (D): Altocumulus are the heap-like clouds having flat bases and rounded tops.
This statement is correct. The term "alto" indicates middle-level clouds, and "cumulus" refers to heap-like or puffy structures. Altocumulus clouds form as patches, sheets, or rolls of individual heap-like cloud elements that typically exhibit relatively flat bases and rounded, roll-like tops.

Therefore, statements (A), (B), and (D) are true, while statement (C) is false.