Which form of energy does a plant store when light is transformed during photosynthesis?
a. chemical energy
b. thermal energy
c. mechanical energy
d. electrical energy

The answer is a. Chemical energy.

Photosynthesis is a process in plants wherein light energy is converted to chemical energy, which is then stored as glucose. This is stored in the chloroplasts.  

Photosynthesis is divided into two stages: the light and dark stage. In the light stage, the light energy is converted into ATP and NADPH, these are like the energy cells of the plant. After the light stage, the ATP and NADPH that were produced are used in the dark stage. The dark stage will use these energy cells to transform carbon dioxide into sugar.