The correct answer is: D. regret
The verb “regret” means to feel sorrow or disappointment over (something that has happened or been done). In the context of the question, the speaker is expressing regret for being late to the party.
The other options are incorrect because they do not accurately reflect the speaker’s feelings. Option A, “will regret,” is in the future tense, but the speaker is expressing regret in the present tense. Option B, “will be regretting,” is also in the future tense, and it implies that the speaker’s regret will be ongoing. Option C, “have been regretting,” is in the past participle form, and it implies that the speaker’s regret has been going on for some time. Option E, “regretted,” is in the past tense, and it implies that the speaker’s regret is over.
In conclusion, the correct answer is “regret” because it is the only option that accurately reflects the speaker’s feelings of sorrow or disappointment over being late to the party.