I have submitted app and it got rejected as it crashed in iOS 9 devices. Can I opt out for iOS9 like I will say my app will support up to iOS 8.4. I don't want to support iOS 9?
No, you can not opt out of the current (or about to be released) iOS version. Apple will test iOS app submissions to their App store on the iOS version currently running on the devices that its customers are currently buying or will soon be buying just days from now. And that's currently iOS 9.0. Why put an app in the store that will crash a couple days after a customer buys it just before upgrading their device?
Also, most apps built with the SDK for the previous iOS version should work on devices running the next iOS version, perhaps after testing (and tweaking API usage) on devices running a beta version of that next iOS version, which is available to all enrolled developers. This testing does require Xcode 7 (although using the Xcode-beta or GM makes some of the testing and debugging a lot easier). There are other ways to put a test app on your test device, such as Ad Hoc deployment via iTunes.