The correct option is D An armada is a fleet of warships.
The article 'an' is used before collective nouns that start with a vowel sound, while the article 'a' is used before collective nouns that begin with a consonant sound. Here, the collective noun 'armada' starts with the vowel sound 'a' and the collective noun 'fleet' begins with the consonant sound 'f'. Therefore, this sentence uses the articles 'a' and 'an' correctly.