The spool pin holds the spool of thread from which the thread is fed into the machine.
Step 1: Analyze the options.
- Bobbin case — Holds lower thread.
- Needle plate — Supports fabric.
- Spool pin — Holds thread spool.
- Presser foot — Holds fabric in place.
Step 2: Conclusion.
Thus spool pin is correct.
Final Answer:
\[
\boxed{(C) \, \text{Spool pin}}
\]