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.

It is a mixture. simply put, a compound must be pure, that is, of only one chemical structure and composition. a mixture is made of different chemical substances (i.e. elements, molecules, compounds) that are not chemically bonded to each other, but instead physically grouped together, as in the case of gasoline (a homogeneous solution).