Answer
We can rank the time constants in order from largest to smallest:
$\tau_b = \tau_c \gt \tau_a$
Work Step by Step
We can write a general expression for the time constant:
$\tau = \frac{L}{R}$
We can write an expression for the time constant in each case:
(a) $\tau_a = \frac{L}{R}$
(b) $\tau_b = \frac{2L}{R}$
(c) $\tau_c = \frac{L}{R/2} = \frac{2L}{R}$
We can rank the time constants in order from largest to smallest:
$\tau_b = \tau_c \gt \tau_a$
