How do I turn off the alerts if I select the wrong answer in practice mode?

By default the Theory Test app will tell you if you have selected the wrong answer and offer to show the correct answer when working through a practice session:

If you prefer not to see whether you've answered correctly until you finish the practice session, touch "Settings" from the main Theory Test screen:

Touch the button to the right of "Alert if selected answer is incorrect" (so that it shows "OFF") and then touch "Main Menu":