Q.Which one of the following statements is incorrect?

The correct option is (A) — the statement about malpighian tubules in both cockroaches and prawns is incorrect.

• Malpighian tubules are the excretory organs of insects such as the cockroach, so that part of the statement is correct.
• Prawns, however, are crustaceans (a different class of Arthropoda from insects), and they excrete through green glands (antennal glands), not malpighian tubules — so lumping cockroaches and prawns together under the same excretory structure is wrong.
• (B) is correct: comb plates mediate locomotion in ctenophores.
• (C) is correct: flame cells carry out excretion and osmoregulation in Platyhelminthes such as Fasciola. …

Going through each statement against the chapter's account of these groups:

• (A) Cockroaches and prawns excreting via malpighian tubules — malpighian tubules are the excretory structures of insects such as the cockroach. Prawns, however, belong to a different class of Arthropoda (Crustacea), and their excretory organs are green glands (antennal glands), located near the base of the antennae — not malpighian tubules. Treating cockroaches and prawns identically here is factually wrong, since the two groups use different excretory structures.
• (B) Comb plates and ctenophore locomotion — ctenophores move by the beating of eight rows of fused cilia called comb plates, exactly as described. This statement is correct.
• (C) Flame cells in Fasciola — Platyhelminthes, the phylum to which Fasciola belongs, carry out excretion and osmoregulation using specialised cells called flame cells. This statement is correct. …

Method: Statement-by-statement verification table

Build a small table pairing each statement's claimed group with its claimed excretory/locomotory feature, and check each claim against what is specifically known about that group, rather than judging the whole statement at once.