Hem stitch is used to finish edges neatly, especially at the bottom of garments.
Step 1: Analyze the options.
- Back stitch — Strong seam stitch.
- Hem stitch — Used for hemming.
- Chain stitch — Decorative.
- Cross stitch — Embroidery.
Step 2: Conclusion.
Therefore hem stitch is correct.
Final Answer:
\[
\boxed{(B) \, \text{Hem stitch}}
\]