Answer: The correct answer for the blank is-
A. tropism.
Tropism is a biological process that indicates growth of a plant in a particular direction in response to any external stimuli (any change in the environment that induces a specific response in a biological system).
For example - Phototropism in plants is a type of tropism process in which plants tend to grow towards the direction of sunlight.
Thus, option A) is the right answer.
