In photosynthesis, redox reactions ultimately transfer electrons from to
a) o2… co2
b) c6h12o6… o2
c) h2o… c6h12o6
d) h2o… co2

The correct answer is: D) H2O… CO2

Redox reactions are reactions of reduction (molecules tend to gain electrons) and oxidation (molecules tend to lose electrons).

Photosynthesis is a set of redox reactions. In photosynthesis electrons are removed from water (H2O) to carbon dioxide CO2 and then used to turn CO2 into organic compounds.  

The first step: Oxygen in water is oxidized (light energy required)

The second step: Carbon in CO2 is reduced; sugar is formed (dark reaction).