The correct option is D both a and b
During secondary growth in dicot stems, the phellogen or cork splits or opens up. This results in the formation of small openings called lenticels.
These have closely arranged parenchymatous cells on the exterior instead of cork cells to form lens-shaped cracks. They are developed in barks due to secondary growth and allow gaseous exchange in woody trees.
They also contribute to transpiration but in tiny amounts, because the suberized complementary cells present underneath the pore restrict unnecessary water loss.
The lenticels do not have any role in photosynthesis as it take lacks chloroplasts which are required for photosynthesis.
So, the correct answer is "option D"