The sentence talks about the process of 'cultivating microalgae'. The blank must be occupied by an adjective/adjective phrase that describes this process. Among the given options, 'efficiently' is an adverb, 'efficiency' is a noun and 'efficient' is an adjective. Option a) contains no adjective; in option d), the adjective 'extreme' incorrectly qualifies the adverb 'efficiently'. Further, option b) is grammatically incorrect, the noun 'efficiency' must be qualified by an adjective, but 'extremely' is an adverb. Option c) is grammatically incorrect; the adjective 'efficient' must be qualified by an adverb, which is not the case here. Option e) is grammatically correct and brings out a meaningful sentence. Hence, e) is the correct answer.
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