When I was a child, I . . . . . . . . to school everyday instead of going by cycle.

The correct answer is A. walked.

The sentence is in the past simple tense, which is used to talk about things that happened in the past and are not happening now. The other options are not in the past simple tense.

• Option B, have been walking, is in the present perfect continuous tense. This tense is used to talk about things that started in the past and are still happening now, or have happened repeatedly over a period of time.
• Option C, had walked, is in the past perfect tense. This tense is used to talk about things that happened before another event in the past.
• Option D, have walked, is in the present perfect tense. This tense is used to talk about things that have happened at some point in the past and have a connection to the present.

In this case, the speaker is talking about something that happened in the past and is not happening now, so the past simple tense is the correct tense to use.