To find the elevation of base camp, we need to subtract the descent from Lisa's starting elevation.
Starting elevation: 457 feet Descent: 221 feet
Elevation of base camp = Starting elevation - Descent Elevation of base camp = 457 feet - 221 feet = 236 feet
Since the resulting elevation is above sea level, we conclude that base camp is 236 feet above sea level.
So, the correct answer is: 236 feet above sea level.