Answer:
b)chemotropism
Step-by-step explanation:
Tropism is the term used to represent the response of an organism or part of it towards or away from stimulus. Tropism also occurs in plants, as they also respond to internal and external stimuli around.
Tropism can be of different types depending on what causes the response (stimulus). However, in the case of the growth of a pollen tube from pollen grain towards an ovule is triggered by a chemical stimuli produced by the ovary. Hence, the pollen tube responds to this chemical stimuli by growing downwards in the direction of the ovule produced by the ovary. Therefore, this is an example of CHEMOTROPISM i.e. chemo-chemical, tropism-response.