Answer
We can rank the materials according to index of refraction:
$d \gt b \gt a \gt c$
Work Step by Step
According to Snell's law:
$n_2~sin~\theta_2 = n_1~sin~\theta_1$
$\theta_a \gt \theta_b$, and so $n_b \gt n_a$
Note that:
$n_a~sin~\theta_a = n_b~sin~\theta_b = n_c~sin~\theta_c$
$\theta_c \gt \theta_a$, and so $n_a \gt n_c$
Note that:
$n_b~sin~\theta_b = n_c~sin~\theta_c = n_d~sin~\theta_d$
$\theta_b \gt \theta_d$, and so $n_d \gt n_b$
We can rank the materials according to index of refraction:
$d \gt b \gt a \gt c$
