Use 'an'
Look at these words:

an orange  an apple  an uncle

an  elephant  an interchange

These words all begin with a vowel (a, e, i, o, u). Such words always take the article an when the sound of the vowel is used in their pronunciation.

Some words beginning with 'h' take an if the 'h' is silent. If the 'h' is pronounced then a is used.

e.g.

Silent 'h':

an hour  an honour  an heir

Pronounced 'h':

a hotel  a horse  a harmonica

The vowel 'u' has two forms--the 'u' of up and the 'u' of union. When the first form is used the noun takes an. When the other form is used the noun takes a.

an umbrella  an undershirt
but
a uniform  a union  a university

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