Translation:Byddwch yn dawel!
The drop-down hints are not context sensitive. The command form of bod is needed here: bydd!/byddwch!. See the course notes for examples and explanations.
To find the course notes generally, go to https://forum.duolingo.com/topic/924/hot and see 'Course tips and notes'. The 'duome' link there is useful for browsing all the notes in one place. We recommend reading the notes for each new section as you start it.
The ti-form of the command is Bydd!
See the course notes for 'Commands 1' and 'Commands 2' - https://www.duolingo.com/skill/cy/Commands-1/tips-and-notes, https://www.duolingo.com/skill/cy/Commands2/tips-and-notes