How to enable Developer Options USB Debugging in Zenfone 5, Zenfone 3, Zenfone 2 or any device

Android operating systems hide developer options in the settings because not every one needs it. However, if you do stuff like rooting and flashing custom ROMs, you'll want to enable the USB debugging mode which can be accessed via developer option. The title says Zenfone 2 and Zenfone 5 but the method to enable this option is actually same for any android device. Zenfone 5 doesn't have official Lollipop update so the first step before you can flash a custom Android Lollipop 5 is to unlock the bootloader. To unlock bootloader you need to use ADB in USB debugging mode.

Update: Now you can easily unlock Zenfone 2's bootloader by using the official bootloader unlock tool.

Enabling developer options and debugging in ASUS Zenfone 2 and Zenfone 5 and other android devices

Go to Settings >> Scroll all the way down and select "About phone (or tablet)"
Scroll down from About and select "Software information"
Tap on the Build number until you get a notification that says "developer option is enabled"
Go back to Settings >> Scroll down and you will now see "Developer options"
Open Developer options and enable "USB debugging" (or Android debugging)

Once inside the Developer options you get access to lots of advanced features like advanced reboot, bug report, Bluetooth HCI snoop, Process Stats, show visual feedbacks for touches, show pointer location, show CPU usage and so on. If you have installed custom ROMs like Cyanogenmod 11, you will also get the option to enable or disable root.

Now that your USB debugging is on, you can proceed to root and unlock your device's bootloader. After that you simply need to flash the latest CWM (clockworkmod recovery) which lets you install the latest compatible custom ROMs in your Android device.