If you choose your master slide in the slide option, it works in a specific way. The slide master follows the design of your own master slide. You can find your master slide in athe following steps. 

When you have powerpoint opened, go to view. This tab is found in the PowerPoint menu on the left side of the ‘’He;[’’ tab (image below of the menu bar) 

Now that you have the ’View’ menu in front of you, please click on the ‘’Slide Master’’ icon. This is found in the master views section of your view menu. (See GIF below) 

As you can see, it is easy to find the Master Slide. It has all the other slides underneath it. It is possible to have multiple master slides in one presentation. This is because there are multiple templates. Each slide with the same template belongs to that one master slide. The question will take the Master Slide layout that is underneath.  

You can also change it from the master slide itself, when you know which one is the master slide. This can be done without going to the menu.