change from passive to active: Our action is based on the assumption that the competition will be taken by surprise.
We based our action on the assumption that we will take the competition by surprise.
To change the given sentence from passive voice to active voice, we need to identify the subject performing the action. In this case, the subject is "we" and the action is "our action." Here's the active voice version:
"We assume that we will take the competition by surprise."