Remark You are starting out with an equation that has no addition or subtraction associated with it. Therefore, you cannot use addition of subtraction to isolate a variable, which makes A wrong. You can only use multiplication or division to isolate a factor.
Argument A A is wrong. See above.
B B is wrong as well. The reason is very important to know in algebra. Let's consider what you are given and what you want.
C = pi * d is what you are given. Pi and d are multiplied together. If you want to have d all by itself, the only way to do it is to do the opposite of multiplication. That's division.
You divide both sides by what is with the factor you want to have alone. You want d. You have to divide by pi. What you get looks like this.
D is the answer because if you look at the question it is saying "C equals pi times d" so, in this case it would be c=pi/d and none of the answers given is showing that, so therefore D is the answer
