[ajanthpraveen]

I guess the confusion is at the word "free protein"... assuming that by free protein one refers to precipitated protein, then option C is correct. Otherwise, as a classical case, option A is right.

Moreover, are they expecting an assumption of progressive addition of ammonium sulphate? (that's another point to ponder)...

Sunil sir, can this queation be challenged. As the question  was not clear.(

ya, me too considered it as precipitated protein (free protein ) and choose option C