Solution:sky-high, coal-black, oil-rich → overactive → grey-green, red-hot: These items belong to the class of compound adjectives, which are multi-word (or multi-morphemic) combinations functioning together as a single adjective modifying a noun.
A. sky-high, coal-black, oil-rich qualify as compound adjectives because each consists of two semantic parts working together to describe a noun, typically hyphenated when used attributively (e.g., a sky-high price, coal-black hair, an oil-rich country).
B. overactive is a closed compound adjective, formed by combining over (functioning as an intensifying bound form) with active to yield a single adjectival unit describing excessive activity (e.g., an overactive child).
D. grey-green, red-hot are classic hyphenated compound adjectives involving adjective + adjective structures used to describe mixed colours (grey-green water) or extreme states (red-hot iron).
(C) contains underestimate and overcook, which are verbs, not adjectives. They express actions rather than qualities.
(E) contains hand-wash and air-condition, which also function as verbs, even though their related nouns (hand-wash cycle, air conditioner) may look adjectival in other contexts.
Thus the expressions that correctly fit the category of "compound adjectives" are A, B and D.