A vine grows sideways, twisting along a railing, What type of tropism is this plant exhibiting?
Thigmotropism, plants that grow towards or away from an object they touch. It's not necessarily growing away from the sun because it could be in an open field. The question doesn't specify.
That must be phototropism especially if the plant is twisting along the rail towards light. However, if it is twisting towards sunlight then that is heliotropism.
Phototropism because it is in reaction with the sun. but it is negative tropisms because of the way it goes away from the sun.