Correct Answer - Option 4 : Eastern side
The sun rises in the east. Thus, if the shadow of the Taj Mahal falls on the Red Fort, implies, the Red Fort must be in the west of the Taj Mahal. Similarly, the sun sets in the west. Thus, if the shadow of the Red Fort falls on the Taj Mahal, implies, the Taj Mahal is to the east of the Red fort.
Hence, the correct answer is the Eastern side.