The correct answer to this question is C, Irrational Number. A number is irrational if it can not be written as a ratio that is complete of two integers. These numbers can not be written down fully because after the decimal point, there is an infinite amount of numbers.
A part of mathematics, irrational numbers are more likely to be found in the subject of Geometry. A rule of thumb with irrational numbers is that the square root of every number that is natural, will either be an irrational number or an integer. Pi is the most known example, as the number for it is 3.14159265358979323.