If a figure has four sides then it cannot be a triangle

This is true. A triangle can only have 3 sides.

Well, 1/3 is equal to 0.3, and 1/4 is equal to .25, so it can't be a.
3/4 is equal to .75 and 6/9 is equal to 0.6, so it can't be b.
5/8 is equal to 0.625 and 20/32 is also equal to 0.625, so it could be c.
7/9 is equal to 0.7 and 21/28 is equal to 0.75, so it can't be D.
Your best bet is C. Because both sides of C are equal.
C 5/8=20/32