I don't regret ________ her what I thought of her husband, even though it might have upset her.
regret doing sth后悔做某事。regret to do sth很遗憾要做某事。根据句子后半部分“即使这可能让她伤心了”可知我应该是已经告诉她了。因此B、C、D项均可排除。