Concept:The phrase "held on" is used to indicate events scheduled on a particular day.Explanation:The sentence requires a verb phrase that correctly pairs with "saturday" (a day)."Lectures" are events, and events are "held on" a specific day."Hold on" (present tense) doesn't fit the passive meaning; "held on" is the correct past participle used in passive voice."Hold at" and "held at" are for locations, not days.Answer:C. held on