A sentence must make perfect sense if non-essential modifiers are removed.

Here, 'the recovery of copper from the drainage water of mines' is a non-essential modifier (a modifier separated by commas).

The sentence now is -
Leeching [as a method of the extraction of minerals], it was well established ...
Note that the part enclosed in the square brackets is a modifier to the noun - Leeching.
The sentence essentially says this -
Leeching, it was well established ... - this sentence is incorrect. (this sentence is similar to this -John, he went to the movies).

A - incorrect.

B - essentially the sentence is this -
Leeching [as a method of the extraction ....], but + Clause.
Note that 'but' requires parallel structure (just like 'and').

Leeching + Modifier cannot be parallel to a Clause - hence, B is incorrect.

C - correct answer. Note that we are referring to a specific point in the past - 18th century.
Hence, past tense 'was' is used.

D - "method of extracting mineral that was as early as the 18th century".
this implies "method that was as early as the 18th century" - absurd.

E - usage of past perfect 'had been' is incorrect.

B - essentially the sentence is this -
Leeching [as a method of the extraction ....], but + Clause.
Note that 'but' requires parallel structure (just like 'and').

Leeching + Modifier cannot be parallel to a Clause - hence, B is incorrect.

Hi Crackverbal ,

Thanks for your explanation.

All clear. Can you please explain more option B?

Many thanks in advance.

Kavin

Whatever follows the 'but' is a clause. 'but' requires parallelism

look here -
but [until about 25 years ago] miners did not realize that bacteria take an active part in the process.

the main clause is this -
'miners did not realize that bacteria take an active part in the process'.

this - 'until about 25 years ago' is a modifier.

1st part of the sentence -
Leaching, [the recovery of copper from the drainage water of mines], as a method of the extraction of minerals well established as early as the eighteen century

I can remove the phrase in square brackets as it is a non-essential modifier.

I am left with this -
Leaching as a method of the extraction of minerals well established as early as the eighteenth century.

this is not a sentence/clause. There is no verb in the clause.
'well established' is a modifier, not a verb. (Because, this is not a sentence - 'Leeching [as a method of extraction] well established as early as 18th century'. Delete the phrase in the square brackets and you will see that it is not a sentence.)

hello
thank you for you help .
can you please explain about D?
I understand that seems method was as early as 18 century but i think method refers to leaching and this is correct....

could you correct me?