The potential barrier existing across an unbiased p - n junction is...

The potential barrier of the unbiased $p$-$n$ junction is $V_0 = 0.2 \text{ V}$.

When the junction is reverse-biased with a voltage $V_R = 0.1 \text{ V}$, the effective potential barrier increases.

The new potential barrier is given by $V = V_0 + V_R = 0.2 \text{ V} + 0.1 \text{ V} = 0.3 \text{ V}$.

To diffuse from the $p$-side to the $n$-side, a hole (with charge $+e$) must overcome this potential barrier.

The minimum kinetic energy required is $K = eV = e \times 0.3 \text{ V} = 0.3 \text{ eV}$.