composer discard changes: what do the [y,n,v,d,s,?] do

Choosing ? shows you the details:

y - discard changes and apply the update
n - abort the update and let you manually clean things up
v - view modified files
d - view local modifications (diff)
s - stash changes and try to reapply them after the update

You can also check the source code, the relevant part is here.