An agent that has a dark brown oily liquid composition and is very persistent and difficult to remove from the skin is referred to as a/an:

a. Nerve agent
b. Vesicant
c. Asphyxiant
d. Radiological agent

b
Rationale: a. Nerve agents may be liquid, vapor, or both and can be inhaled or absorbed.
b. Correct.
c. An asphyxiant is an inhaled gas that interferes with respiration.
d. A radiological agent emits ionizing radiation.