Why are some elements nuclei unstable?

Iwould say the answer is d

1) what is light? light is an electromagnetic wave visible to the human eye.  2) what is a reflection? the rebounding of a light ray when it hits a mirror.3) what is the incident ray? contacts the surface of an object.4) what is the reflected ray? bounces off a reflective surface.5) what is the normal ray? perpendicular to the surface of a mirror.6) what is the angle of incidence? an angle formed by the incident ray and the normal.7) what is the angle of reflection? an angle formed by the reflected ray and the normal.8) what is refraction? (bending.)9) what is a converging lens? bring together light rays that pass through.10) what is a diverging lens? spreads out light passing through.11) focal point? the real point where the refracted rays meet when the incident rays run parallel.

25,000 = 2.5 x 10⁴250 = 2.5 x 10^20.025 = 2.5 x 10⁻²0.00025 = 2.5 x 10⁻⁴

Could you display the mirrors