If the dark period is interrupted by a flash of red light of 660...

Short-day plants require a continuous critical dark period for flowering. The photoreceptor responsible for measuring the dark period is phytochrome, which exists in two interconvertible forms: $P_r$ (absorbs red light at $660\text{ nm}$)$and P_{fr}$ (absorbs far-red light at $730\text{ nm}$).

During the dark period, $P_{fr}$ slowly converts back to $P_r$. If the dark period is interrupted by a flash of red light ($660\text{ nm}$), $P_r$ is rapidly converted to $P_{fr}$, which inhibits flowering in short-day plants.

This inhibitory effect of red light can be reversed if it is immediately followed by a flash of far-red light ($730\text{ nm}$). The far-red light converts the $P_{fr}$ back to $P_r$, thereby nullifying the effect of the red light flash and allowing the short-day plants to flower.

Answer: far red light of 730 nm