We have to identify the rational function among the given functions.
Rational function is a function that is the ratio of two polynomials. It is rational because one polynomial is divided by the other polynomial, like a ratio.
1. Consider the first function 
 , since it is not a function that is the ratio of two polynomials. So, it is not a rational function.
, since it is not a function that is the ratio of two polynomials. So, it is not a rational function.
2. Consider the second function 
 , since it is not a function that is the ratio of two polynomials. So, it is not a rational function.
, since it is not a function that is the ratio of two polynomials. So, it is not a rational function.
3. Consider the third function 
 , since it is not a function that is the ratio of two polynomials. So, it is not a rational function.
, since it is not a function that is the ratio of two polynomials. So, it is not a rational function.
4. Consider the fourth function 
 , since it is a function that is the ratio of two polynomials (x+2) and (5x). So, it is a rational function.
, since it is a function that is the ratio of two polynomials (x+2) and (5x). So, it is a rational function.
So, Option D is the correct answer.