How do I choose an appropriate scope for a master's thesis in mathematics?
To expand on scaaahu's very valid comment, consider the following:


what specific topic(s) interest you the most.
which of these do you have some level of confidence in pursuing at a Master's level.
Who has the expertise to be able to advise you, are the willing to do so (they may be overloaded).
Are their enough resources available to start research in the chosen field.
Finally, what is the 'end-game', where do you hope to go with this qualification?