The kind of radiation emitted in the reaction is beta ray.
Explanation:
In the given reaction Thorium which has a mass number 234 and atomic number 90 decays to element with mass number 234 and atomic number 91. Here atomic number is increased by 1.
Atomic number is the number of electrons or protons. Thus release of one electron increases the atomic number of the other product by 1. This is an example of β^-decay. If a positron is released instead of electron the decay is called β^+decay.
