Next button not getting activated for interactions/interconnected slides.

Here's your trigger for the Next button:

It's set to run "when the timeline ends" on the slide. The timeline will end long before the user has a chance to click all of the options. So that trigger will never run. 

However, a "when timeline starts" trigger actually runs every time the user visits a slide. That's true even when the slide is set to "Resume saved state." So all you have to do is edit that trigger so it runs when the timeline starts on the slide. 

Also, you can delete all of the triggers that change the pictures to Visited. Any object with a Visited state will automatically change to Visited when clicked.

• You can find out more about the built-in states here: https://community.articulate.com/discussions/discuss/primer-take-advantage-of-built-in-states/777134 

One more thing: the Visited states for the pictures are slightly lighter than the Normal states. But it was hard for to tell the difference, especially because the change happens immediately before jumping to another slide. So I suggest you make the difference more obvious.