Just adding a simple button is interactive, but it's not engaging the learner. They have not been asked to think about anything other than tapping a button. No learning here!
In this example, I am giving the learner a choice, asking them to make a decision based on what they have learned. Both buttons will take them to the next slide and explain the correct answer, but they have been asked to think. Therefore, I engaged them. This is a simple but effective way to add engagement.
Something as simple as a survey question can increase retention of knowledge learned. (By the way, the answer is the Comstock Lode in Virginia City, NV.) Try different types of questions--drag and drop, multiple answer, etc. to keep things interesting.