My research on the matter can be summarized in a sentence: "It [sporangium] can be composed of a single cell or can be multicellular" (Source: https://en.wikipedia.org/wiki/Sporangium). Yet there shouldn't be a reply placed between "They are" and "They aren't" test options, speaking of "Are bryophyte sporangia multicellular?". A link to the source where I could ascertain whether the bryophyte sporangia is multicellular (if I could ascertain) is highly appreciated.
In Embryophyta (land plants), including bryophytes, the sporangium is usually a multicellular structure.
Perhaps you meant to ask about the number of spore mother cells (SMCs) in each sporangium? That varies across groups. In bryophytes, each sporangium has many SMCs, and accordingly produces a large number of spores. (Contrast this with angiosperms, where a megasporangium [called an ovule] has only one megaspore mother cell.)
References and further reading:
By LadyofHats. (Public domain; https://commons.wikimedia.org/wiki/File:Hornwort_structures.jpg)