Step 1: Drift velocity and potential difference.
In electric conduction, the drift velocity \( v \) of free electrons in a conductor is directly proportional to the applied potential difference \( V \). This is a consequence of Ohm's law, which states that current \( I \) is proportional to the potential difference \( V \), and since current is related to drift velocity, the drift velocity follows the same relationship.
Step 2: Explanation of the relationship.
The drift velocity \( v \) is given by the formula \( v = \mu E \), where \( \mu \) is the mobility of the electrons and \( E \) is the electric field, which is related to the potential difference by \( E = \frac{V}{L} \), where \( L \) is the length of the conductor. Hence, \( v \) is proportional to \( V \).
Step 3: Conclusion.
Thus, the correct relationship is that drift velocity is proportional to the potential difference \( V \).
\[
\boxed{\text{Correct Answer: Proportional to V}}
\]