At what stage do 60% of angiosperms shed their pollen grains?

The correct answer is Option (1) → 2-celled stage

A mature pollen grain contains two cells: a large vegetative cell with abundant food reserve and a small generative cell. In over 60 per cent of angiosperms the pollen grains are shed at this 2-celled stage. In the remaining species, the generative cell first divides by mitosis to form the two male gametes, so the pollen is shed at the 3-celled stage.

NCERT Extract:
Stamen, Microsporangium and Pollen Grain – “… When the pollen grain is mature it contains two cells, the vegetative cell and generative cell (Figure 1.5b). The vegetative cell is bigger, has abundant food reserve and a large irregularly shaped nucleus. The generative cell is small and floats in the cytoplasm of the vegetative cell. It is spindle shaped with dense cytoplasm and a nucleus. In over 60 per cent of angiosperms, pollen grains are shed at this 2-celled stage. In the remaining species, the generative cell divides mitotically to give rise to the two male gametes before pollen grains are shed (3-celled stage).”