Sudhanshu is as much older than Kokila as he is younger than Praveen.
Nitin is as old as Kokila.
Which of the following statement is wrong ? 

Answer: D. Praveen is not the oldest.

In terms of age we have :
Kokila < Sudhanshu,  Sudhanshu < Praveen,  Nitin = Kokila.
So, the sequence becomes :
Nitin = Kokila < Sudhanshu < Praveen.
Clearly, Praveen is the oldest.
Hence, (d) is the incorrect statement.