We use as + adjective/adverb + as to make comparisons when the things we are comparing are equal in some way. Here, the adjective used should be in positive degree. Since 'faster' and 'fastest' are comparative and superlative degree adjectives respectively, they are negated.
'Fasting' is neither an adjective nor adverb.
'Fastly' is incorrect as it is not the valid word in English.
Among the given options, the adverb 'fast' can be used to replace the underlined part.
The correct sentence is: Riya ran as fast as a cheetah and saved the child from falling.
Hence, option (c) is the answer.
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