Well it's not really a matter of which is "best", one is right and the others are wrong :)
[tex] \pi [/tex] is an irrational number, meaning that it can't be expressed with a fraction between integers: in other words, there doesn't exist two integers [tex] a,b [/tex] such that
[tex] \dfrac{a}{b}=\pi [/tex]
This is why option B is wrong. Also, imaginary numbers (option C) are complex numbers with no real part. Since
[tex] \pi = \pi + 0i [/tex]
[tex] \pi [/tex] is not imaginary. Finally, irrational numbers are real numbers, so option D is wrong as well.
