Πis the ratio of to

a)
area to diameter

b)
perimeter to area

c)
radius to diameter
eliminate

d)
circumference to diameter

Pi is explicitly defined as the ratio of a circle's circumference to its diameter. It doesn't matter how big the circle is or what units you use. You always get the same number back, Pi. One of the most inaccurate versions of Pi probably comes right out of the Bible. 1 Kings 7:23-26 says "And he made the molten sea of ten cubits from brim to brim, round in compass, and the height thereof was five cubits; and a line of thirty cubits did compass it round about." That implies Pi=3. The Romans also used to use 3 1/8 because it was presumably easier to calculate on the fly than than the better approximation of 22/7 = 3 1/7. Some other cultures used sqrt(10). You can find a lot of great examples in Petr Beckman's book "The History of Pi".
So, in conclusion, your answer is D) Circumference to diameter.
Hope this is of great help to you, and happy studying~!
~Mistermistyeyed.