The Choice block allows you to offer the user a choice in the form of a clickable button. This choice can be single or multiple : you can add as much as propositions needed by double clicking on the block, then on the "+" icon. Each propositions can be named as you can input text on them. You can then link the different out of each choice to continue the story.
The Choice block is quite dynamic. For example, you can connect Alter Choice blocks on the left entries of the block and for each corresponding proposition. For more informatio, see this article : http://help.celestory.io/en/articles/3978854-alter-choice
You can also set a timer on the Choice block. You just need to input the number of second allowed to the countdown in the proper section, but you can also link a numeric variable to its left entry. A "timeout" exit will appear, and you will need to link it to the path dedicated to the situation in which the player did not choose in time.