Direction: Choose the most appropriate qualifying word from the options given…

Concept: In English, many adjectives combine with one habitual intensifying adverb by fixed collocation, not with any adverb that merely fits the dictionary meaning. The correct choice depends on which category of adjective is being intensified: adjectives about density, quantity, or weather pair with "heavily"; adjectives about harm or damage pair with "badly"; adjectives about opinions, positions, or the senses pair with "strongly"; and adjectives about resolve or a physical grip pair with "firmly."

Application: "Crowded" describes a state of density -- how full of people a place is. The standard, exam-recognised collocation for describing dense crowding is "heavily crowded" (compare "heavily populated," "heavily congested," "heavily loaded"). Applying this to the sentence gives: "The road was heavily crowded during the festival season," which is both grammatically correct and idiomatic.

Checking each option against its own typical collocation pattern confirms only one fits a density adjective like "crowded":

• "badly" collocates with harmful or negative outcomes (badly injured, badly damaged, badly needed) -- not with density.

• "heavily" collocates with density, quantity, and weather (heavily crowded, heavily populated, raining heavily) -- matching "crowded."

• "strongly" collocates with opinions, positions, and sensory intensity (strongly believe, strongly opposed, strongly scented) -- not with density.

• "firmly" collocates with decisiveness or physical grip (firmly believe, firmly held, firmly gripped) -- not with density.

Therefore, "heavily" is the qualifying word that correctly completes the sentence.