Enter responsive design mode missing

I just upgraded safari to version 9.0.3, one of my parners in another city reproduced an error on my website using responsive desing mode and iOS 9.1 iPhone user agent in a MacBook (OSX 10.9.5).

They were explaining me how to reproduce the issue by using the "Enter responsive design mode" in the develop tab but no matter what I do, that specific option never shows up. I already disabled all extensions and deleted my preferences (not sure if this helps). I tried the option + command + R shortcut but it doesn't do anything neither. I already tried reseting safari and restarted several times.

Where did this feature go?

Edit: added screenshot:

Edit: 2nd screenshot:

According to Apple's Documentation, you have to be running OS X 10.11 El Capitan or above in order for Enter Responsive Mode.

With OS X El Capitan, Safari brings new tools and capabilities that let you create a better surfing experience with the Safari web browser.

Source: https://support.apple.com/kb/PH21491?locale=en_US

As you are running OS X 10.9 Mavericks, you are not able to enable it.

Thus, you need to upgrade to OS X 10.11 El Capitan in order to enable Responsive Mode.