Hello!
Gulp. Where do I start?
I don't think this is a question that needs to be ansered by claculations, but rather an excercise about knowing the orders of magnitude of the speed units!
The speed of sound is roughly 1000 km/h at low level (and this includes the 15000ft in the question). Therefore Mach 0.4 is 400km/h. The only value in the list that comes close to that figure is 200kt.
If you want to now the exact answer you can also google for "mach number calcuator".
Happy landings
max