Here scribe 2 of 𝓟 initially wrote the wrong verb form, understood, which he immediately crossed out, and then wrote inline the correct verb form, answered. But in all of this, scribe 2 forgot to insert the necessary passive auxiliary be. And Oliver Cowdery apparently missed it when he proofed 𝓟 against 𝓞 for this passage. The 1830 typesetter added the necessary be. The use of the be is undoubtedly correct since similar language is found nearby in Mosiah 27:14 (“that the prayers of his servants might be answered according to their faith”) as well as further away in Mormon 9:37 (“that their prayers may be answered according to their faith”).
Summary: Accept the 1830 typesetter’s decision in Mosiah 27:16 to supply the helping verb be in the clause “that their prayers may be answered”.