What is a water-reactive substance?

What is a water-reactive substance?
 
  a. A substance that suffers no chemical reaction with water but is likely to become spontaneously flammable, emit flammable or toxic gases, or generate sufficient heat to self-ignite or cause the ignition of nearby combustible materials
  b. A substance that, by its chemical reaction with water, is likely to become spontaneously flammable, emit flammable or toxic gases, or generate sufficient heat to self-ignite or cause the ignition of nearby combustible materials
  c. Any material that is hydrated beyond saturation
  d. A material that must be stored in a moist atmosphere or it is likely to become spontaneously flammable, emit flammable or toxic gases, or generate sufficient heat to self-ignite or cause the ignition of nearby combustible materials