Unio's excretory system comprises the pericardial gland (Keber’s organ) and one pair of kidneys (Bojanus organ).
A large glandular reddish-brown mass in front of the pericardium is the keber of the pericardial gland. It helps excrete waste products and then disposes them through the kidneys into the pericardial cavity.
Therefore, if it is considered that Assertion is correct then reason must be incorrect.
Hence, the option (C) is the correct choice.
Unio's excretory system contains a pericardial (Keber's) gland of the pair of kidneys (Bojanus organs). Coelomic cavities are part of the kidneys.
Bojanus organs are located on either side of the vena cava under the pericardium. There are two parts for every kidney. It opens into the pericardial cavity by means of the renopenicardial aperture and has a brown, thick and glandular wall. Thin walled and non-glandular is the dorsal part of the kidney, called a urinary bladder. It opens through a small kidney pore into the suprabranchial chamber of the inner gill. A wide opening connects the urinary bladders of both sides. Cilia is on the inner side of the bladder.
Note: Keber’s organ in Unio is located in front of the pericardium and aids in the removal of nitrogenous wastes. However, the Bojanus organ is the one which plays a crucial role in excretion.