Answer:
The correct answer is: past.
Explanation:
Each verb has four principal parts: present, present participle, past, and past participle.
In our case, the present would be: walk, present participle would be: (am, is, are) walking, past would be: walked and past participle would be: (have, has, had) walked.
The principal part, this is, the present is the main form of the verb which can be found in a dictionary. It's actually the base we are using to create other verbal tenses of one verb.
