In the following sentence, there is a missing article. Select the missing article from the options given below:
He works in____ old office building.

Either 1 or 2

The correct answer is Option (3) → Either 1 or 2

We speak about a particular building, so we use the definite article the. The word old starts with a vowel, so the article an can be used if we are not talking about a particular building.