Step 1: Recall the principle of fed-batch fermentation.
In fed-batch fermentation, fresh substrate is added gradually instead of all at once.
Step 2: Identify how substrate inhibition is prevented.
Continuous or controlled feeding prevents the substrate from accumulating to inhibitory levels.
Thus, the substrate concentration is maintained at a low optimum level.
Hence,
\[
\boxed{\text{By maintaining a low substrate concentration}}
\]
is the correct answer.
Therefore,
\[
\boxed{(B)}
\]
is the correct answer.