The horn-shaped sporophyte grows from an archegonium embedded deep in the gametophyte. The sporophyte of a hornwort is unusual in that it grows from a meristem near its base, instead of from its tip the way other plants do. Unlike liverworts, most hornworts have true stomata on their sporophyte as mosses do. The exceptions are the genera Notothylas and Megaceros, which do not have stomata. The sporophyte of most hornworts are also photosynthetic, which is not the case with liverworts.

Answer this question based on the passage: where does the sporophyte of a hornwort develop? Okie... think carefully first, then make a decision:
The relevant information is: The horn-shaped sporophyte grows from an archegonium embedded deep in the gametophyte. So, the answer is an archegonium embedded deep in the gametophyte.