Step 1: Role of heater-treater.
In crude oil processing, a heater-treater is used to separate water and oil. The electrostatic grid provides an electric field inside the vessel.
Step 2: Mechanism.
The electric field induces charges in water droplets suspended in the crude oil. Oppositely charged droplets attract each other, causing them to coalesce (merge). Larger droplets settle faster, enhancing water–oil separation.
Step 3: Evaluate options.
\begin{itemize}
\item (A) Sand separation is done by desanders, not electrostatics.
\item (B) Correct: The grid promotes water droplet coalescence.
\item (C) No energy generation occurs here.
\item (D) Cathodic protection is unrelated to heater-treaters.
\end{itemize}
Final Answer:
\[
\boxed{\text{promote coalescence of water droplets}}
\]