Answer
$ a=b=c$
Work Step by Step
Let $V$ be the potential of battery and $R$ be the resistance of bulb.
potential across $a$ is $V$, therefore
$P_a = \frac{V^2}{R}$
As $b$ and $c$ are in parallel potential across both will be same and equal to $V$. Thus
$P_b = \frac{V^2}{R}$
$P_c = \frac{V^2}{R}$
Since brightness(intensity) is proportional to power. Hence order of brightness is
$a=b=c$
