an mtb
because it's wrong and people never used to say it that way.
The form an for the indefinite article is used before a spoken vowel sound, regardless of how the written word is spelt. If you say 'an otel' when speaking (which is now often regarded as distinctly old-fashioned), then it may be appropriate for you to write 'an hotel'; but most people say 'hotel' with a sounded 'h', and should write 'a hotel'.
By contrast, words such as 'honour', 'heir' or 'hour' in which the 'h' sound is dropped are written with 'an'. Americans who drop the 'h' in 'herb' may also prefer to write 'an herb', but in standard British pronunciation the 'h' is sounded, and 'a herb' is therefore correct in writing. Because 'European' is said with an initial 'y' sound, which counts as a consonantal sound in English speech, it is said (and written) with 'a' not 'an'. An abbreviation such as M.P., which is pronounced em pea, begins with a spoken vowel, and so it is 'an M.P.'
