The correct answer is We brought a picnic lunch when we toured the monuments in Washington, D.C. In this sentence, the verb is bought, which is the past tense of bring. The verbs in the other sentences are regular verbs because they form their different tenses according to an established pattern, in this case, to make the past and past participle, the particle –ed was added to the verb: walk-walked; plan-planned. Irregular verbs do not follow an established pattern, thus, here the past tense of bring is not bringed but brought.