what will happen if we place salt water on a induction cooker?

The induction cooker works by passing a rapidly changing magnetic field through the thing to be heated.

That rapidly changing magnetic field creates an EMF (Electro Motive Force) field. (Faraday's law). This is like a voltage. Current will flow, proportional to the conductivity. No conductivity, no current. Low conductivity, low current. High conductivity, high current. Heat will be produced according to I^2 R.

So there will be a current in the salt water, and it will be heated. But not very much. Its conductivity is much less than copper or iron, so it will not have a very large current or amount of heating.