To calculate Helen's elevation at base camp, we start with her initial elevation of 33 feet above sea level and then subtract the 46 feet she descends.
Starting elevation: 33 feet above sea level
Descent: 46 feet
Calculation: \[ 33 - 46 = -13 \text{ feet} \]
The negative result means she is below sea level. So, the elevation of base camp is 13 feet below sea level.
The correct answer is d. 13 below sea level.