Hi!
The answer would be B. Positive
Step-by-step explanation:
A positive feedback mechanism is that in which a stimulus induces an output which in turn functions to magnify the same response (acting as a stimulus to itself).
As in this case, Ethylene acts as a stimulus for fruit ripening (response). When it is released during fruit ripening, it stimulates nearby fruits to ripen (response), which in turn acts to stimulate further fruits to ripen (response), resulting in the effect being widespread (magnified)
Hope this helps!