Final answer:
The sentence 'They climbed up and down the hill quite fast' is written in the past tense, which is used for actions or states that occurred in the past.
Step-by-step explanation:
The provided sentence 'They climbed up and down the hill quite fast' is written in past tense. Past tense is used for actions or states that were true in the past, but are no longer the case. In English, regular verbs use the base form of the verb plus '-ed' to indicate the past tense, though there are also many irregular verbs that do not follow this pattern. In the example sentence, 'climbed' is the past tense form of the verb 'climb'. No other verb tenses (a. present tense, c. future tense, or d. present continuous tense) would fit this particular sentence because the action of climbing has already happened.
Learn more about tenses