The correct option is
A Venter wall of archegonium
The embryonic sporophyte is often protected by a calyptra, a covering that develops from the venter wall of the archegonium and a part of the gametophyte generation. The venter is the basal swollen region of the stalk of the mature archegonium. This is responsible for the formation of the calyptra.
Thus, the correct answer is option A.