What is the lethal concentration, 50 kill?

What is the lethal concentration, 50 kill?
 
  a. The amount of a substance, measured in milligrams per body mass, that kills half of a group of laboratory animals to which the substance is administered during a preestablished time
  b. The concentration of a substance, measured in parts per million, that kills half of a group of laboratory animals to which the substance is administered during a preestablished time
  c. The concentration of a substance, in milligrams per kilogram (mg/kg), that kills a laboratory animal within 48 hours
  d. The concentration of a substance to which workers can be exposed continuously for a period of approximately 15 minutes without suffering irritation, chronic or irreversible tissue damage, or narcosis sufficient to increase the risk of accidental injury, impair self-rescue, or materially reduce work-related efficiency