a(n) is a high energy electron released from a nucleus during radioactive decay.
alpha
gamma
positron
beta

beta

Explanation:

A beta particle (symbol β) is a high-speed and high-energy electron which is emitted by radioactive decay of the atomic nucleus during process of the beta decay.

Strontium-90 is material which is most commonly used to produce the beta particles.

Beta particles are used to treat conditions such as eye as well as bone cancer.

i believe you are right for all except the 3rd ( look at sqdancefan his answer is much better)