Which sentence is punctuated correctly?

a.
my father has studied these languages: russian; german; and arabic.

b.
on their vacation trip to europe, the richardsons visited vienna, austria; munich, germany; and warsaw, poland.

c.
the dates of the big conventions are may 18-20; july 13-15; and august 17-19.

d.
james city county; virginia, st. mary's county; maryland, and york county; maine are some of the oldest counties in the united states.