The ophthalmologists' offices have begun selling prescription goggles to their customers once the nearby gymnasium had completed adding an Olympic size outdoor pool.
A. The ophthalmologists' offices have begun selling prescription goggles to their customers once the nearby gymnasium had completed adding an Olympic size outdoor pool
B. The ophthalmologists' offices began the selling of prescription goggles to their customers once the nearby gymnasium had completed adding an Olympic size outdoor pool
C. That ophthalmologists have begun the selling of prescription goggles to their customers once Olympic pool the nearby gymnasium had completed adding
D. The ophthalmologists began selling prescription goggles to their customers once the nearby gymnasium had completed adding an Olympic size outdoor pool
E. That ophthalmologists' offices began selling prescription goggles to their customers once the nearby gymnasium had completed adding an Olympic size outdoor pool
Explanation: Here we can see a 3/2 split if we want to solve as quickly as possible
One is Opthamalogists' offices vs Opthalmologists
We have a pronoun reference "their customers" in the subsequent line. So, it refers to opthalmologists.
In C & D --> we can go to D because C has issue with have begun & fragment sentence ( once Olympic pool the nearby gymnasium had completed adding)