Answer
C. $P>S=T> Q=R$
Work Step by Step
since all bulbs are same(resistance) so brightness will be proportional to potential difference across bulb.
potential difference across each bulb is
$V_P =\frac{2}{3}V$
$V_R =\frac{1}{3}V$
$V_Q =\frac{1}{3}V$
$V_S =\frac{1}{2}V$
$V_T =\frac{1}{2}V$
hence order of brightness is
$P>S=T> Q=R$