What is the principal part of walked?

base, past, past particle, or present particle

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.