Having discovered the child prodigy John Klein to be suffering from at

Hi Devavrat,
Below is the sentence.

Having discovered the child prodigy John Klein to be suffering from attention deficit disorder (ADD), it is now thought that this was a major cause of his anger and recurrent clinical depression.

Before seeing the answer choices, let's try to understand the meaning of the sentence. The sentence is trying to show a cause and effect. It says that they have found the child prodigy John Klein to be suffering from attention deficit disorder (ADD)((CAUSE)) & Then it is now thought that this was a major cause of his anger and recurrent clinical depression. ((EFFECT)).

Now let's get to POE.

(A) Having discovered the child prodigy John Klein to be suffering from attention deficit disorder (ADD),
This is incorrect because HAVING has to be followed by the doer of the sentence, but nothing is mentioned.

(B) To have discovered the child prodigy John Klein to be suffering from attention deficit disorder (ADD),
This sentence is incorrect TO HAVE DISCOVERED is a verb ed modifier, which again should be followed by DOER but nothing is mentioned.

(C) Since historians have discovered that the child prodigy John Klein suffered from attention deficit disorder (ADD),
This choice is correct. This mentions HISTORIANS and there is a proper cause and effect mentioned in the passage.

(D) Since the suffering of attention deficit disorder (ADD) by the child prodigy John Klein was discovered by historians,
Passive voice is not that admired in GMAT especially when a better option is mentioned.

(E) Due to the fact that the child prodigy John Klein was suffering from attention deficit disorder (ADD) was discovered by historians,
This sound illogical. Due to the Fact is incorrect and changes the meaning.

IMO C.
Hope this helps.