"You are thinking very highly about Prashant but he is not so" means

The original sentence "You are thinking very highly about Prashant, but he is not so" implies 'you' overestimate Prashant (ie you think he's better than he actually is), but reality contradicts your high opinion ("he is not so").

Only option A matches this meaning: "You have a good opinion about Prashant but he is not as good as you think" perfectly captures the contrast between perception and reality.

The other options fail, because "Prashant is much better than what you think"(option B) is opposite meaning (says he's better than you believe); "Your view is philosophical" (option C) is Irrelevant to the original message, and "Prashant is as good as you think" (option D) contradicts "he is not so."