The correct option is A the reddest
The phrase in option B is superlative, with the definite article and the superlative form of 'red', hence it's the correct option. Option A is incorrect because when a single syllable positive is changed to a comparative, if the noun ends with a single consonant preceded by a short vowel, then that consonant is doubled, and to the noun 'red' we double the consonant and add the suffix 'est', making it 'reddest'. Option C is incorrect because the superlative 'most' is not used for single syllable nouns and the noun is a positive. Option D is incorrect because the article is missing and the superlative of red is spelled wrong.