Explanation

(c) Potential difference across $1\ M\Omega$ resistor is
$V_{P} - V_{B} = \frac{18V \times 1 \times 10^{6}\Omega}{(0.2 + 1) \times 10^{6}\Omega} = \frac{18V \times 1 \times 10^{6}\Omega}{1.2 \times 10^{6}\Omega} = 15V$
$V_{B} = - 15V$ [Given]
$\therefore V_{P} - V_{B} = 15V$ or $V_{P} = 15V + V_{B}$
$= 15V - 15V = 0V$
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Potential difference across $200\ k\Omega$ resistor is
$V_{A} - V_{P} = \frac{18V \times 0.2 \times 10^{6}\Omega}{(0.2 + 1) \times 10^{6}\Omega}$
$= \frac{18V \times 0.2 \times 10^{6}\Omega}{1.2 \times 10^{6}\Omega} = 3V$
$V_{A} = + 3V$ [Given]
$\therefore V_{A} - V_{P} = 3V$ or $V_{P} = V_{A} - 3V$
$= + \ 3V - 3\ V = 0\ V$