Thank you for the opportunity to help you with your question!
Diamond has the highest melting point here because it is a network covalent crystal. The main component that determines melting point is intermolecular forces. Network covalent forces are strongest, next would be NaCl, which is held together by ionic forces. Next, H2O, held by hydrogen bonds. And finally Kr, which is just held by London Dispersion Forces (weakest).
Please let me know if you need any clarification. I'm always happy to answer your questions.