Explanation

(b) After one $\alpha$- emission, the daughter Nucleus reduces in mass number by 4 unit and in atomic number by 2 unit. In $\beta$- emission the atomic number of daughter nucleus increases by 1 unit.
The reaction can be written as
$\ _{92}U^{238}\overset{- 8\alpha\ \ \ \ }{\rightarrow}\ _{76}X^{206}\overset{- 6\beta\ \ \ \ }{\rightarrow}\ _{82}Y^{206}$
Thus, the resulting nucleus is $\ _{82}Y^{206}ie,\ \ _{82}{Pb}^{206}.$