Answer: B
To find out which of these figures is a polygon, let's look at the definition of a polygon.
A polygon is a closed figure whose sides are line segments that do not cross.
Using the definition above, let's go and look for which figure is a polygon.
A would not be a polygon because the segments cross.
C would not be a polygon because the segments cross.
D would not be a polygon because it's not closed.
This leaves our answer which is B.