Rational numbers are natural numbers.?
1.)always
2.)sometimes
3.)never

Rational numbers are 2.) sometimes natural numbers.

For a number to be rational, it has to be able to be written as a ratio, such as 1/2 or 3/1. 
For a number to be natural, it has to be a whole number. So, 3/1 could be is natural and rational, while 1/2 is only rational.