Why photosynthesis is a type of …

• Redox reaction (oxidation-reduction reactions) are marked by the transfer of electrons. During oxidation, a substance tends to lose its electrons whereas, during reduction, a substance tends to gain electrons.
•  Photosynthesis is a process in which plants containing chlorophyll convert the carbon dioxide into sugars in the presence of sunlight by a set of redox reactions. 
• Calvin cycle represents a set of reactions which are used to remove electrons from water which are then used to turn carbon dioxide into organic compounds.
• Redox reactions during photosynthesis :

It involves two major steps - 

• The first step is the oxidation in oxygen in water molecule in the presence of light

2H₂^​O → O₂​ + 4H⁺
The second step is the reduction step where hydrogen ions reduce the carbon in carbondioxide
4H⁺ + CO_2​ →  (CH_2​O) + H_2​0
The net reaction is
CO₂​ + H₂​O → (CH_2​O) + H₂​O