How to Get an A* in A-Level Maths

A good multiple choice exam question is written with plausible wrong answers in mind - so that the question can identify people who don't have a full understanding of the subject.

Even this tiny bit of knowledge can be used to our advantage.

So - here are the answers to a multiple choice question... which is right?

a) 4π cm²
b) 8π cm²
c) 16 cm²
d) 16π cm²
e) 32π cm²

(π means pi by the way)

* I must admit that this problem is taken from The Art of Strategy - a book I'm reading applying Game Theory to real world situations. Recommended for those who want to apply maths to economics, business or politics!