What is stimulus ?

What is the stimulus in: (a) phototropism? (b) geotropism? (c) chemotropism? (d) hydrotropism? (e) thigmotropism?

While conducting experiments to study the effect of various stimuli on the plants, it was observed that the roots of a plant X grow and bend towards two stimuli A and B but bend away from a third stimulus C. The stem of the plant X, however, bends away from stimuli A and B but bends towards the stimulus. C.The stimulus B is known to act on the roots due to too much weight of the earth. Keeping these points in mind, answer the following questions: (a) What could stimulus A be? (b) Name the stimulus B. (c) What could stimulus C be? (d) The branches of a fallen tree in a forest grow straight up in response to two stimuli. What could be these two stimuli out of A,B and C? Also name these two stimuli.