Question

this is what defines an element as radioactive. a how long it exists b the proton - electron ratio c the fact that it is a natural element d the fact that it gives off high energy particles

Explanation:

Radioactive elements are defined by their ability to emit high - energy particles. Option A (how long it exists) is related to half - life but not the defining characteristic. Option B (proton - electron ratio) is more about atomic structure in general. Option C (being a natural element) is not the key. The key is the emission of high - energy particles.

Answer:

D. The fact that it gives off high energy particles