Answer:
B. People with mental disabilities.
Step-by-step explanation:
The gas-killing vans were gas chambers in vans that can be taken wherever required. Such vans were a mobile version of the killing chambers of the Holocaust extermination under Nazi Germany.
These vans were reequipped with chambers like the gas chambers in the concentration camps. And it was used to kill people by carbon monoxide poisoning. The test subjects of such chambers were the patients of mental disability, leading the way for a faster means of execution rather than transporting the victims to the stationary gas chambers.
Thus, the correct answer is option B.