The correct answer is: walked.

Correct sentence

When we were young, we walked a mile to school every day.

Why “walked” is correct

  • The phrase “when we were young” shows we are talking about the past.
  • We also have “every day” , which shows a habitual action in the past.
  • For a past repeated habit, we use the past simple :
    • we walked a mile to school every day.

Why the other options are wrong

If the choices are:

  • walk
  • walked
  • walking

Then:

  • walk → present tense, does not match “when we were young”.
  • walking → participle (used in continuous forms like “were walking”), but here we need a main verb in past simple, not a continuous form.

Do you want a brief explanation of how to talk about habits in the past using past simple vs “used to” vs “would”?