The correct option is B an
'A' and 'An' are indefinite articles which are used before unspecific, singular countable nouns. 'The' is a definite article and is used to refer to specific things.
'A' is used before words beginning with a consonant sound.
'An' is used before words beginning with a vowel sound.
In the given sentence, the word 'aeroplane' begins with a vowel sound, and it is unspecific. Therefore, the correct option is B) an.