"Developer" option is not available under Settings menu. Why?

Solution 1:

In versions of Xcode prior to Xcode 6, you can connect the device to your computer over USB, open Xcode → Organizer and select Use for Development.

Use for Development was removed from Xcode in Xcode 6 and later. It is no longer possible to do this for devices using Xcode 6 or later, even with the Devices page (⇧⌘2).

Solution 2:

I found that by powering off the device, and turning it back on while connected to the machine running Xcode 6.3, the Developer settings menu returned for my iOS 8.3 devices.

Solution 3:

Apple removed the device list from Organizer in Xcode 6. Try going to Window->Devices. You should see your attached device in the list and from there you can enable development.