How do I get rust stain clean, which is in my bathroom sink,

try a paste of baking soda and hydrogen peroxide

Try WD-40

Bar Keepers Friend gets most rust off of metal in sinks. It can be found where you buy comet or ajax cleansers.

Bar Keepers did a pretty good job for me, but didn't remove the stain entirely. You can also try bleach -- let it soak on the sink a few minutes, scrub as best you can, and rinse well with warm water as you continue to scrub while it is rinsing.

You can buy a rust remover for this. Then figure out why it is staining. Is the faucet dripping? Maybe it needs a new washer. Has the porcelain finish worn off? Get a little bottle of porcelain paint and paint just that area. Try to fix the underlying problem that is creating the stain.

CLR (Calcium, Lime and Rust) Soak a rag with this (wear gloves) and let it sit for about an hour, then follow up with Barkeepers Friend and a Magic Eraser and see if that doesn't take care of the issue. (I have used regular white out or white appliance touch up paint for porcelain chips in my sinks and tubs)