"She was sorry she didn't attend her friend's wedding." "Yes, she ........."

Answer: Option D

"Was regretting that she couldn't go" is the correct choice because it uses the past progressive tense, which is appropriate to express an ongoing feeling of regret about a past event.

Option A: "regretted she can't have gone" is incorrect because the phrase "can't have gone" is grammatically incorrect in this context.

Option B: "was regretting she does not go" is incorrect because it mixes tenses improperly ("does not go" is present tense, while "was regretting" is past tense).

Option C: "will regret she didn't go" is incorrect because it refers to a future regret, while the sentence clearly discusses a past event.

Thus, the correct choice is Option D: "was regretting that she couldn't go."