Which type of activity does an author use to develop the plot?

I believe you're really asking "Where do authors get their ideas?"

The plot is just what happens in the story, and authors make those up from their imaginations. Everybody has ideas all the time - writers are just people who take those ideas and make up stories with them. They ask questions like "What would happen if ___?" and "Why does that happen?" and "What if _____?"

Once you have a good idea, you make it into a story plot by thinking logically and figuring out how your characters would deal with the situations you put them into, then what would happen next, and so on.