Final answer:
The beta particle is the type of radiation most commonly emitted by the isotopes used in biological contexts and by those isotopes that are important biologically.
Step-by-step explanation:
The form of radiation emitted by the most commonly used isotopes and by most biologically important isotopes is a beta particle, which is symbolized by the Greek letter β. A beta particle is essentially an electron that is ejected from the nucleus and has a 1- charge. The process of beta decay results in a neutron being converted into a proton, keeping the mass number unchanged but increasing the atomic number by one. This form of radioactive emissions is more common than alpha particles (α, which are essentially helium nuclei) and gamma radiation (high-energy electromagnetic radiation), though all these forms are observed in various radioactive decay processes.