To determine what substance W is in the context of a chemical reaction that takes place in the chloroplasts of plant cells, we should consider the process of photosynthesis. In photosynthesis, plants use sunlight, carbon dioxide, and water to produce glucose and oxygen.
From the options you provided:
a) Sunlight - This is essential for photosynthesis as it provides the energy needed for the reaction.
b) Starch - This is a product of photosynthesis, not a starting material, so it wouldn't fit as W.
c) Methane - This is not related to the photosynthesis process in chloroplasts.
d) Minerals - These are important for plant growth but are not directly involved as a substance in the photosynthesis reaction.
Given these options, substance W is most likely:
a) Sunlight
because it is a crucial component in the photosynthetic process occurring in the chloroplasts.