Answer: 

Step-by-step explanation:
Since, according to the property that rational numbers are closed under addition.
If a and b are the rational number then (a+b) must be a rational number.
Since, In First option 
Addends √ 9 and √ 9 are rational number.
Also, 2 √9 is the rational number.
Thus It is following the property.
In Second option 
Addends
and
are rational numbers.
Also,
is a rational number.
Thus, It is also following the property.
Note: Both √6 and √2 are the irrational numbers.
This is why we did not take them.