English question, please help?

its IN, coz though the ducks appear to be ON the river, they are technically IN, due to their legs,. and river has got no surface of its own; the ducks are ON the river surface. (matter of location, and the case of grammar actually) ON should be used where there is a flat surface =)

p.s: am a language teacher, in case u need some assurance.. XX

both are fine.

I can see ducks on the river.

Unless they are literally under the water, then they would be in it, but otherwise they are simply on the river.

The correct answer is:
I can see ducks in the river.

In the river

I think it may be 'in'