Understand the disease.

• Onchocerciasis, also known as river blindness, is a parasitic disease caused by the nematode Onchocerca volvulus.

• A. Chloroquine is an antimalarial drug.
• B. Ivermectin is an antiparasitic drug that is highly effective against the microfilariae of Onchocerca volvulus and is the drug of choice for mass treatment programs for onchocerciasis.
• C. Metronidazole is an antibiotic and antiprotozoal drug, used for bacterial and some protozoal infections, but not onchocerciasis.
• D. Zidovudine is an antiretroviral drug used to treat HIV/AIDS.

Ivermectin is the drug commonly used in the mass treatment of onchocerciasis.

The final answer is $\boxed{\text{B. Ivermectin}}$