The correct word to fill the blank is "apologized," which is the correct form of the verb to indicate that the seniors expressed regret for their actions.
- "Apologising" is the present participle, which doesn't fit the sentence structure.
- "Apologetic" refers to being regretful, but it doesn't fit grammatically in the sentence.
Use the past tense form "apologized" to show an action that has already taken place.