Answer:
My brother lives in an apartment in the city centre
Step-by-step explanation:
'an' comes before the word 'apartment' because it begins with a vowel. It would be 'a' if it began with a consonant like 'a flat'.
'The' comes before city because it is a specific. 'The' is a part of speech called 'an article' and it works when the subject is known or specific. The city they refer to here, is not any general or unspecific area, but a city that is known by the writer.