Step 1: Understanding the disease.
Coccidiosis is a protozoan disease affecting poultry, caused by a group of protozoan parasites from the genus Eimeria.
Step 2: Analyzing the options.
(A) Favus: Incorrect. Favus is a fungal infection, not a protozoan disease.
(B) Pullorum: Incorrect. Pullorum is a bacterial disease, not a protozoan infection.
(C) Ranikhet: Incorrect. Ranikhet disease is caused by a virus, not a protozoan.
(D) Coccidiosis: Correct. Coccidiosis is a protozoan disease affecting poultry.
Step 3: Conclusion.
The correct answer is (D) Coccidiosis, as it is the protozoan disease of poultry.