raghupara
carcass
Tricky but if you focus on the meaning the answer appears clear.
The subject is metallurgy ,
filler, then HERE we need a VERB not as so A and B are out.
E doesn't make sense because had been give the meaning that the process is not used anymore. So C and D. In D we have moulding metal but is wrong : the process is about moulding not metal: metal moulding is correct.
C is the best answer

Kudos if is clear and useful my post
Hi Carcass,
Please correct my understanding here.
I stick with E- had been, because there is a but introduced in the sentence showing a contrast that only since a few decades, people have started to realize the action of bacteria and probably no manual refinement/metallurgy is required.
Sorry, if this has been pathetically viewed.
Thanks,
Raghupara
@Raghupara, If we go with E, then Sentence looks like:
Metallurgy, the process of refining metals from naturally mined ores,
had been a method of metal moulding, well knownas early as the medieval period, but until about several decades ago workers did not realize that bacteria take an active part in the process.
where
Clause 1 = Metallurgy, the process of refining metals from naturally mined ores,
had been a method of metal moulding, well known as early as the medieval period
Conjunction = but
Clause 2 =but until about several decades ago workers did not realize that bacteria take an active part in the process.
Let us see Clause 1 as independent clause
Metallurgy,
the process of refining metals from naturally mined ores,
had been a method of metal moulding, well known as early as the medieval period.
The green part in above sentence can be taken away leaving only the below sentence.
Metallurgy
had been a method of metal moulding, well known as early as the medieval period.
The blue portion in above sentence is unfortunately referring to Metal moulding rather than Metallurgy, which is improper reference.
HTH