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

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 ?

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