The correct option is C Tegmen
The outer thick layer is called as testa and inner thin one as tegmen. The seed develops from the tissue, that is the integument which originally surrounds the ovule. The seed coat forms from the two integuments or outer layers of cells of the ovule, which derive from tissue from the mother plant, the inner integument forms the tegmen and the outer forms the testa. The seed coats of some mononocotyledon plants, such as the grasses, are not distinct structures, but are fused with the fruit wall to form a pericarp.
Therefore, the correct answer is option C.