Final answer:
The option 'emits flammable gas' is not classified as a pre-defined explosive type. Explosive classifications include the potential to detonate or violently change chemically, which does not apply to substances that simply emit flammable gas.
Step-by-step explanation:
The student has asked which of the following is not classified as a pre-defined explosive type: a) Organic Peroxides, b) Self-Reactives, c) Emits Flammable Gas, d) Self-Heating. According to the classification of hazardous materials, emits flammable gas is not a predefined explosive type. Explosives are assessed based on their ability to detonate or undergo violent chemical changes, where they can be rated on a scale from 0 (stable) to 4 (may detonate). Organic Peroxides and Self-Reactives have the potential for explosive decomposition, whereas Self-Heating substances can spontaneously ignite under certain conditions. However, 'emits flammable gas' refers to a substance that releases gas that can be flammable when it comes in contact with air but is not an explosive on its own.