3) What evidence indicates that the nuclei of strontium-90 atoms are unstable?
A)
Strontium-90 electrons are in the excited state.
B)
Strontium-90 electrons are in the ground state.
C)
Strontium-90 atoms spontaneously absorb beta particles.
D)
Strontium-90 atoms spontaneously emit beta particles.

D)  Strontium-90 atoms spontaneously emit beta particles.

Explanation:

The evidence that indicates that the nuclei of strontium - 90 atoms are unstable is that they spontaneously emit beta particles.

For every atomic nucleus, there is a specific proton to neutron ratio which ensures stability of the nucleus.

Any nucleus with a neutron/proton combination different from its stability ratio will be unstable and split into one or more other nuclei.

Strontium  - 90 atoms are an example of this. To attain stability, they emit beta particles spontaneously.