这是it强调句型,

强调时间状语从句"not until man had learned how to make iron and steel"

not和until作为句型搭配用法，往往是分开的，但是如果要强调，则会合在一起，即：

It is/was not until...that...此时，主句的否定结构到了强调句中药变为肯定结构。比如：1.His father won't come back until midngiht every day.

It is not until midnight that his father will come back every day.

2.His father didn't come back until midnight yesterday.

It was not until midnight that his father came back yesterday.

答案是D

the construction of various machines was made possible

（主语+主语补足语）

把它变为主动语态就是 People made "the construction of various machines" possible. make+宾语+形容词做宾补 

直到人类学会如何生产钢和铁,各种机器的建造才成为可能.“leran how to make  iron and steel"动作早于“the construction of various machines was made possible”，所以not until 从句用过去完成时，主句用过去时。