Explanation

(b)
The fourteen elements which follow actinium from thorium($Z$=90) to lawrencium ($z$=103) are called actinoids. They involve the filling of 5$f$-subshell. Californium (Cf) has the atomic number 98 and its configuration is as
$Cf(Z = 98):\lbrack Rn\rbrack 5f^{10},6d^{0},7s^{2}$
Hence, it is a member of actinide series.