Answer: option D. Response to environment.
Justification:
Organisms respond to their environment in a variety of ways.
The environment interacts with the organisms via stimulus and the response to such stimulus is named tropism.
The example of the question, the plant's stem bends toward the window, is called positive phototropism, which means that the plant growth response to light such that the stems bend toward light to permit the leaves to receive more light.
The plant's stems posses positive phototropism and negative gravitropism.
Gravitropism is the response of the plant growth to gravity.
Stems grow away from gravity meaning that this is negative gravitropism, but roots grow toward gravity which is positive gravitropism.