Final answer:
Californium is likely the radioactive element among the choices given, as it is a synthetic, highly radioactive element unlike the more stable, naturally occurring erbium, calcium, and selenium.
Step-by-step explanation:
Out of the options provided, Californium is probably radioactive. Californium is a synthetic element and is known for its high radioactivity. Elements like erbium, calcium, and selenium are naturally occurring and generally not radioactive. Specifically, calcium is one of the Alkaline Earth Metals and is found in huge deposits on Earth, making it abundant and stable rather than radioactive.
When discussing radioactivity, it's crucial to understand that certain elements are inherently unstable and release energy in the form of particles or electromagnetic waves to reach a more stable state. This process is what makes an element radioactive. The synthetic elements, such as those in the actinide series including californium, are typically known for their radioactive properties due to their large, unstable nuclei.