Here $\mu_v=1.523$

$\mu _R= 1.5145$

Mean refractive index

$\mu =\large\frac{1.523+1.5145}{2}$

$\qquad= 1.51875$

Dispersive Power w is given by

$w= \large\frac{\mu_v-\mu_R}{(\mu-1)}$

$\quad= \large \frac{1.523-1.5145}{(1.51875-1)}$

$\quad =0.1639$

Hence b is the correct answer.