Use property names of the objects that are in your collection.
Or [this] if you want the object itself to be displayed.
QuotedHtml is a property of the object, that is passed to the reply/forward builder template. You can use TemplateKeys property to include additional key/value pairs in the template.
ReplyBuilder replyBuilder = new ReplyBuilder(original);
replyBuilder.TemplateKeys.Add("XYZ", "abcd");
replyBuilder.TextReplyTemplate = @"[Text] template text [XYZ] [QuoteText]";
replyBuilder.HtmlReplyTemplate = @"[Html] template html [XYZ] [QuoteHtml]";
replyBuilder.Html = "reply-html";
replyBuilder.Text = "reply-text";
MailBuilder builder = replyBuilder.Reply(new MailBox("bob@example.com"));
IMail mail = builder.Create();
Assert.AreEqual("abcd", mail.Subject);
Assert.AreEqual("reply-text template-text abcd > original-text.", mail.Text);
Assert.AreEqual("reply-html template-html abcd \r\n original-text.\r\n ", mail.Html);