Plz tell me whether the given answer is incorrect or not , becoz according to NCERT the statement is correct....

Dear Student

The first statement is incorrect.
That is because, excretion in prawns takes place by Green glands or Antennary glands. 

P.S. You are correct regarding excretion in cockroaches. It takes place by Malphigian tubules. However, excretion in prawns takes place by Green glands.

Regards