Indefinite articles ‘a’ or ‘an’ are used when the noun is not specific. ‘Mount Everest’ is a proper noun and cannot be said to be an unspecific thing. Therefore indefinite articles cannot be used with it; hence Option B and C are incorrect. As per the rules of use of articles, definite article ‘the’ can be used before a proper noun when it is qualified by an adjective or a defining adjectival clause. In this question, the adjective ‘tallest’ is qualifying the noun ’mountain’ but ‘Mount Everest’ is not qualified by any adjective, thus we cannot use the Option A ‘the’ with it. Also as per the rules of use of articles, the use of article is omitted before most proper nouns like names of people, continents, countries, cities, individual mountains, islands etc. therefore we can say Option D ‘No article’ is the correct answer.