Question

Which point $a$ or $b$ is at the higher potential?

Answer 1

At steady state, the capacitors are fully charged and thus it does not allow to pass d.c through them and hence they act as open switch at steady state. Also as the switch is open, thus no current flows through the circuit.

⟹ $V_a=18V$ and $V_b=0V$

⟹ Point $a$ is at high potential w.r.t b.