Step 1: Recall what Nylon-6 is.
Nylon-6 is a man-made polymer of the polyamide family, the same family as natural silk and wool.
Step 2: Understand the question.
We need the single starting unit (the monomer) that joins up many times to build Nylon-6.
Step 3: Look at the name clue.
The "6" tells us the monomer has 6 carbon atoms. This rules out monomers with the wrong carbon count.
Step 4: Identify the monomer.
Nylon-6 is made from caprolactam, a ring-shaped molecule with 6 carbon atoms.
Step 5: See how it reacts.
On heating with a little water at high temperature, the caprolactam ring opens and the units link into long chains.
Step 6: Conclusion.
So the monomer for Nylon-6 is caprolactam. Note that Nylon-6,6 instead needs two different monomers, which is a different case.
\[ \boxed{\text{Caprolactum (Option 3)}} \]