Organisms using protonephridia for excretion are…(a)flatworms, rotifers, earthworm (b)amphioxus, cockroach, prawns(c)planaria, cephalochordates, rotifers(d)rotifers, cephalochordates, annelids

The correct answer is (c) planaria, cephalochordates, rotifers.

Explanation:

Protonephridia are a type of excretory system found in some simple organisms. These are primarily used to remove waste products from the body. Protonephridia are mainly seen in organisms that lack specialized kidneys, like flatworms (e.g., planaria), rotifers, and cephalochordates.

Planaria (Flatworms): These organisms possess a protonephridial system, which helps them excrete waste and maintain osmoregulation.
Cephalochordates (e.g., Amphioxus): They also use protonephridia for excretion, performing a similar function of waste removal and osmoregulation.
Rotifers: These tiny aquatic organisms also possess protonephridia for excretion.
The other options involve organisms that use different types of excretory systems:

Option (a): Earthworms use a different system called nephridia for excretion, not protonephridia.
Option (b): Amphioxus and prawns do not use protonephridia for excretion. Prawns, for instance, use antennal glands.
Option (d): Annelids (like earthworms) do not use protonephridia for excretion; they have nephridia instead.
Thus, the organisms using protonephridia are planaria, cephalochordates, and rotifers.