Our office building comprises (1)/ seven floors and a restau rant at the top in an (2)/ area of about eight hundred sq. me tres. (3)/ No Error (4)

Answer

2

Explanation

within: inside or not further than a particular area or space.
Look at the sentence :
People who live within the city pay higher local taxes than people who live just outside the city.
Hence, seven floors and (with) a restaurant at the top within an is the right usage.