When cells perform photosynthesis, they transform energy from one form to another. Which of the?

Energy is not a physical unit. It is a philosophical concept and an accounting technique used to analyze mechanical and chemical exchanges. In nuclear physics it is defined as a wavelength of light, in biology it is a synonym for metabolism, and in public utilities they say energy when they mean connectivity. Anybody who uses the word in any other context does not know what he is talking about.

In photosynthesis, plants convert CO2 and water to sugar, using red light to power the transformation.

You forgot to include the rest of the question.