The parent element decays by beta decay. which daughter element will form?

Β decay can happens in two ways as β⁻ decay and β⁺ decay.

β⁻  decay is a conversion of neutron into a proton by emission of an electron.
If β⁻ decay happens, then the daughter nucleus will have same number of mass. But the number of protons will be increased by 1 and number of neutrons will be decreased by 1 compared to parent nucleus.

β⁺ decay is a conversion of proton into a neutron by emitting a positron.
If β⁺ decay happens, then the daughter nucleus will have same number of mass. But the number of protons will be decreased by 1 and number of neutrons will be increased by 1 compared to parent nucleus.

C. Decomposition

Since one reactant turns (decomposes) into two products, it is a decomposition reaction