which of the following correctly explains why the formula for the circumference of a circle is 2 pi r. Pi is defined as the ratio of the circumference of a circle to the diameter of that Circle.

Let 
C---------> the circumference of a circle
D---------> diameter of a circle
r----------> radius of a circle

we know that
C=2*pi*r
D=2*r
so
C=D*pi-------------> pi=C/D
Pi is defined as the ratio of the circumference of a circle to the diameter of that Circle
when you multiply the diameter in both numerator and denominator
pi*D=(C/D)*D------> pi*D=C
and the diameter is two times the radius
so
D=2*r
C=pi*D------> C=pi*(2*r)-----> C=2*pi*r

therefore

the answer is the option D
Two times r equals the diameter of the circle. Pi equals the circumference divided by the diameter. When you multiply, the diameter is in both the numerator and the denominator, which cancels out, leaving the circumference