[tex]1.999999\inf[/tex]
So adding a rational number, like 2, it would be irrational still:
[tex]2+1.999999\inf=3.999999\inf[/tex]
That being said, answer C is wrong
Now, what is 2/2? Or 4/1? Whole numbers are one of the 3 types of rational numbers.
[tex]2\ /\ 2=1\\\\4\ / 1=\ 4[/tex]
Thus the answer appears to be D
Now lets use the process of elimination to double check the answer:
The wording of your question is what will help us out here:
Which of the statements using operations on rational or irrational numbers always results in a rational number?
Because of that, any of the choices that involve a irrational number can result with another irrational number like so:
[tex]a)\ 1.999\inf+ 1.999\inf=3.888\inf\\\\b)\ 2.999\inf / 1.999\inf= 1.50025\inf[/tex]
And there you go, the only answer that hasn't been proven wrong is answer D.