The most significant visual difference between an F-15A and an F-15C is in the main landing gears. The A-model Eagle's main landing gears are vertical and that made the aircraft tricky to land in crosswinds on account of the narrow landing gear track. The C-model's main landing gear legs are angled outward 4 degrees, while seemingly insignificant, is sufficient to keep the Eagle from tipping to one side when landing in a crosswind.
Source: F-15 Eagle Engaged: The World's Most Successful Jet Fighter by Steve Davies and Doug Dildy. Osprey Publishing, 2007. "Active Duty Eagle Units in the Cold War" p115.