What is required for the daily task: “complete daily challenge”?

You need to reach the third level reward in any daily challenge (the highest). Check the daily reward each hour (they change) and try to find one that you can complete. It's not easy reaching the top level reward.

Don't bother with 3rd level reward in the hourly bonus. You'll spend 3x more value in rss and minute boots than the reward is worth. I've calculated it to be 1/3 to 1/4 the value of what you spend or used to get the reward. It's just a ploy to get you addicted and to spend real money to get goals using in-app purchases. Avoid that goal.