A is incorrect. A semicolon is used to separate two clauses that are major clauses (ie both could stand alone). A semicolon has the same meaning as "and." the words after plant make a list. The list in this case is
roots
stem
leaves
flowers (which should be plural).
B is correct.
C is actually a tough one. When I was taking this grammar, we were taught to use a colon if it was a business letter and a comma if it was friendly. A semicolon is not correct according to my usage.
D is the same as A. There is a list involved. You should be using a colon.
