Step 1: Recall the function of plasma cells.
Activated B lymphocytes differentiate into plasma cells.
These plasma cells synthesize and secrete large quantities of antibodies (immunoglobulins).
Step 2: Identify the correct cell.
Therefore,
\[
\boxed{\text{Plasma cells}}
\]
are responsible for antibody production.
Hence,
\[
\boxed{(D)}
\]
is the correct answer.