Which single substance in the above equation is repeated in raw material as well as reproduced as an end product?

Glucose produced during photosynthesis is soon polymerised into starch. What does polymerisation mean?

What is the source of oxygen released in photosynthesis — CO₂ or H₂O ?

Dark reaction involves utilisation of CO₂ in producing C₆H₁₂O₆ (glucose). Why is this phase called dark reaction ?