The hormone that promotes internode/petiole elongation in deep water rice is Ethylene.

Ethylene is a plant hormone that plays a critical role in growth and development, including responses to environmental stimuli. In the case of deep water rice, when the plants are submerged, the ethylene concentration increases and promotes rapid internodal elongation, which allows the plant to keep its leaves above the water surface.

So, the correct answer is Option B - Ethylene.