[Notice about the above graphs:  On the far left end, 
the x-coordinates are all , and on the right
end they are , as those are partly cut off the
pictures.]

A is clearly periodic.  The first period
starts at  and ends at .
Then at  it starts repeating that exact same 
figure and ends it at .  Then it starts again at
 and it is assumed top repeat that figure over
and over indefinitely.

C is clearly periodic. At  it starts repeating
the same figure as the figure between  to .

D is very clearly periodic.  It repeats the same v-shape
over and over.

B is NOT periodic because it does not repeat the same figure
at all.

Edwin