Well it's not really a matter of which is "best", one is right and the others are wrong :)
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
such that

This is why option B is wrong. Also, imaginary numbers (option C) are complex numbers with no real part. Since

is not imaginary. Finally, irrational numbers are real numbers, so option D is wrong as well.