Which suffix indicates the past tense of verbs?

The suffix that indicates the past tense of verbs is -ed. This suffix is commonly added to regular verbs to denote that an action has already occurred. For instance, the verb "play" becomes "played," which clearly indicates that the action took place in the past.

The other options represent different aspects of verb forms. The -ing suffix denotes the present participle or gerund form, which indicates ongoing actions (e.g., "running"). The -s suffix is used to show the third person singular present tense (e.g., "he runs"). The -en suffix typically forms the past participle of some irregular verbs (e.g., "driven" from "drive"), but does not function as a general marker for past tense. Therefore, -ed is the clear and correct choice for indicating past tense in verbs.