Final answer:
Sodium is a dangerously reactive metal that is hazardous due to its reactivity with water and air, not because it's inherently flammable or combustible.
Step-by-step explanation:
Sodium is a very reactive metal that reacts explosively with water and vigorously with air. It is considered a hazardous substance due to its high reactivity rather than due to any flammability or combustibility. When sodium comes into contact with water, it produces hydrogen gas which may ignite spontaneously if enough heat is generated. However, on its own, sodium is not categorized specifically as a flammable solid nor as a combustible liquid. Therefore, the correct answer to the student's question, 'Sodium is a _________,' would be A) dangerously reactive material.