Oh, Dr. Seetal, you are posting a series of classic questions! I wonder if you should make some flash cards containing such questions; you are soooooo good at making flash cards!!! (I have created a special "Seetal" folder in my Step 1 Notes archive in my PC
Look at the following mnemonic:
Cancer is the latin name for CRAB
& multiple myeloma is a common type of hematologic cancer in the elderly. The monoclonal cell line that overdevelops in multiple myeloma is the plasma cell (answer D is correct). The proteinaceous casts consist of light Ig chains, aka Bence-Jones protein.