(b) stomium

Explanation:

When the capsule of the sporangium in Dryopteris matures, about 4 lower median cells of the jacket stretch tangentially. The two median ones identify the place from where the capsule opens. This is called as stomium. The other cells of the jacket develop a thickening . At maturity, the indusium dries exposing the sorus. Due to loss of water in the annulus, the upper walls contract and inner ones straighten, the annulus coils. This feature exerts pressure on the wall resulting in the breaking of the capsule, between the cells of the stomium, thereby, releasing the spores.