Answer:
B. Photosynthesis
Step-by-step explanation:
The process in purple arrow indicate the process of photosynthesis.
Photosynthesis is carried out by green plants and it involves the use of carbon dioxide and water in the presence of light to synthesize glucose. The light energy needed for the reaction is absorbed by the green component of plants, the chlorophyll.
The process is summarized by the equation:

Hence, the purple arrow represents the diffusion of carbon dioxide into the vegetation as a precursor for photosynthesis.
The correct option is B.