forms - Should I hide continue button until tasks are completed?

In a multi-step wizard, some of the steps present the user with a list of tasks that must be completed before continuing to the next step.

Each task in the list opens a popup where the user completes the task.

I have three possible solutions for the continue button:

a) Show it all the time. If the user clicks it before all tasks are complete the user will be shown information about what must be done before he can continue.

b) Show a disabled button, that is enabled when all tasks are completed. In its disabled state, the button will have some generic message that all tasks must be completed before you can continue.

c) Hide the button and only show it once all tasks are completed.