The lethal dose, 50 kill, or LD50 best defines

The lethal dose, 50 kill, or LD50 best defines
 
  a. The concentration of a substance that kills half of a group of laboratory animals to which the substance is administered during a preestablished time.
  b. The limit of a concentration to which an average healthy person can be exposed to a toxic substance during an 8-hr day.
  c. The amount of a substance that kills half of a group of laboratory animals to which the substance is administered during a preestablished time.
  d. The maximum dosage to a substance to which workers may be exposed continuously for a time period of 15 to 30 minutes.