Plants, like every other living organism, responds to external stimulus. In this question, the stimulus is LIGHT from the sun entering through the window sill. The plant is responding positively to the light stimuli by growing towards the direction where the light is coming from. This exhibit is called PHOTOTROPISM.
Phototropism, comes from two words viz photo- meaning light and tropism- meaning movement. Hence, phototropism is the growth of an organism (plant in this case) in response to a light stimulus. The case described in this question is POSITIVE PHOTOTROPISM because the growth occurs towards the light source.
