Explanation

(c) Here $p - n$ junction is forward biased. If $p - n$ junction ideal, its resistance is zero. The effective resistance across $A$ and $B$
$= \frac{10 \times 10}{10 + 10} = 5k\Omega.$
Current in the circuit $I = \frac{V}{R} = \frac{30}{15 \times 10^{3}} = \frac{2}{10^{3}}\ A$
Current in arm $AB = I = \frac{2}{10^{3}}$
Potential difference across $A$ and $B = \frac{2}{10^{3}} \times 5 \times 10^{3} = 10\ V.$