To find the elevation of base camp, we start with Helene's initial elevation of 27 feet above sea level and subtract the 32 feet she descends.
The calculation is as follows:
\[ 27 , \text{feet} - 32 , \text{feet} = -5 , \text{feet} \]
This result means that base camp is 5 feet below sea level.
Thus, the correct response is:
5 feet below sea level.