Apositive tropism is a plant's growth response the stimulus.
a. away from
b. toward
c. diagonal to
d. opposite of

B. Toward

Tropism is a descriptive reaction of a certain stimuli on how an organism responds to the confronted stimuli which can be called positive tropism or negative tropism.
In positive tropism as the proposed answer toward  involves the organism leaning forward or direct to the acting stimuli whereas the negative tropism is exactly the opposite. 

What's the yellow wallpaper?