I have been serving in Gomal University________ fifteen years

since

for

none of these

I have been serving at Gomal University for fifteen years

میں گومل یونیورسٹی میں پندرہ سال سے خدمات انجام دے رہا ہوں۔

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The word 'for' is used to show a period of time, in the past, present, or future.