Step 1: Recall hybridoma technology.
Hybridoma technology produces monoclonal antibodies by fusing antibody-producing B lymphocytes with immortal myeloma cells.
Step 2: Identify the fusion agent.
The most commonly used chemical for cell fusion is
\[
\boxed{\text{Polyethylene glycol (PEG)}.}
\]
Therefore,
\[
\boxed{(C)}
\]
is the correct answer.