Which is the most common type of embryo sac in angiosperms ?

In a majority of flowering plants, one of the megaspores is functional while the other three degenerate. Only the functional megaspore develops into the female gametophyte (embryo sac). This method of embryo sac formation from a single megaspore is termed monosporic development.

Functional megaspore undergoes three sequential mitotic divisions to form a mature female gametophyte.