Answer: an
Step-by-step explanation:
When using an article in English, a simple rule is followed. For the words that start with a consonant sound, the article "a" is used while "an"before is applied before words that begin with the vowel sound.
In the case of MBBS, we will realize that the letter "m" starts with a vowel sound when it is said in isolation. Therefore, we will make use of "an".