I am sure that what he said is true.

=I believe that what he said is true

A. that   B. about that   C. of that   D. that what

解析：D为正确答案。

在I am sure that+从句结构中，that引导形容词宾语从句，形容词that宾语从句又嵌套/包含一个what引导的主语从句。

以上宾语从句的语法结构解析如下

(1)that what he said is true为主句的宾语从句

(2)what he said为that宾语从句主语成分

(3)is为that宾语从句系动词

(4)true为that宾语从句表语成分

备注(1)：that宾语从句的主语成分由what he said从句充当。

备注(2)：从句后面必须是具有至少1套“主语+谓语”结构的句子，以上的that宾语从句后面的主谓+谓语 结构中，只不过把主语以“what从句(what he said)形式充当了”谓语由“系动词(is)+表语(true)充当了” 这种英语中十分常见语言现象。

2. We all know the truth that wherever there are air, water and sunlight there are living things.

A. where   B. wherever   C. that   D. that wherever

解析：D为正确答案

that引导同位语从句，同位语从句有嵌套一个由连词wherever引导的地点状语从句。(that同位语从句是“名词truth”的同位语）

以上同位语从句的语法结构解析如下

(1)that wherever there are air, water and sunlight there are living things.为主句同位语

(2)living things为that同位语从句的主语成分

(3)are为that同位语从句的系动词

(4)there为同位语的there be句型引导词

(4)wherever there are air, water, and sunlight 为that同位语从句的地点状语

备注：同位语从句地点状语由wherever从句充当。