答案：D

考查代词。假若选A或B，那么转换成陈述句即为：This is you want to say. / That is you want to say. 显然句中的两个谓语动词 is与want相冲突。选D组成的句子是 Is that all you want to say?其中的that 为句子主语，all 为表语，you want to say为定语从句，用以修饰all。