The error is in option (c). Here 'considerably' will be used in place of 'considerable'. The use of the word 'considerable' disturbs the meaning of the sentence since 'considerable' means 'notably large in size, amount, or extent.' It is an adjective and can be used only to describe a noun or a pronoun but not a verb. In this case, to qualify the verb 'reducing', an adverb needs to be used. The correct usage, thus, is the adverb 'considerably' which means ' to a notably large extent or greatly', conveying the meaning of the sentence perfectly. Hence, (c) is the right answer.
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