Some Rivers are Ponds. All Ponds are Ocean. No Ocean is Sea. Examine the conclusions: I) Some Ponds are…

The correct choice is: Only III.

Conclusion I does not follow: all ponds are oceans and no ocean is a sea, so no pond is a sea. Conclusion II does not follow: only some rivers are ponds (and so oceans); the other rivers need not be oceans.

Conclusion III follows as a possibility. The rivers that are ponds are oceans and so cannot be seas, but nothing is said about the other rivers, so some of them may be seas. Hence 'Some Sea may be a River' is possible, and by the usual rule for 'may be' conclusions it follows. So only III follows, and 'None of conclusions I, II and III necessarily follows' is wrong.