The $\begin{array}{l}238\\ \text{\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}U}\\ 92\end{array}$disintegrates to give 4a and 6b particles. The group number of the daughter element is 

No. of a=4  i.e. 4 x 4 = 16 A = 238-16 = 222

No. of b = 6  i.e. (4x2)= 8-6=2  Z = 92 – 2 = 90.

i.e. ${90Th}^{{ }^{222}}$ III or 3rd group