Sentence Correction

Metallurgy, the process of refining metals from naturally mined ores, as a method of the moulding of metals, it was well known as early as the medieval period, but until about several decades ago workers did not realize that bacteria take an active part in the process.

(A) as a method of the moulding of metals, it was well-known

(B) as a method of the moulding of metals well-known

(C) was a well-known method of metal moulding

(D) was a well-known method of moulding metal that was

(E) had been a method of metal moulding, well-known

Is there any best approach to determine the answer?

I think that the correct option is D. But I don't know how to give a good explanation of this.

Any expert could help me. I'd really like an explanation here. Please.