Photosynthesis requires a few main components. Mainly, Sunlight, Water and Carbon dioxide.
For this reaction, the reactants are
1) Water
2) Carbon dioxide
For water, it enters the cell by Osmosis. Water enters a cell through a partially permeable cell surface membrane.
When water enters a space through a partially permeable membrane from a high water potential to a low water potential, the process is called osmosis.
For carbon dioxide, it enters the cell through diffusion.
Diffusion involves particles moving from a region of higer concentration to a region of lower concentation. Therefore, the answer is Diffusion.
Extra knowledge:
Carbon dioxide dissolves into the thin film of moisture on the surface of the spongy mesophyll layer for easy transportation to the cells involved in photosynthesis.
