It should be natural selection since there is no external influence implied. (like environment or genetic mutations.)
Answer:
The yellow leaf was not exposed to the sun so it can't produce sugar.
Explanation:
The yellow region of the leaf didn't produce sugar because it wasn't exposed to sunlight. Part of the leaf was exposed to the sun, thus this leaf produced sugar, while the yellow part of the leaf did not because it was not exposed to the sunlight.
True....................................................................
