Answer:
Option D
Step-by-step explanation:
Rational number is a number which can be expressed in the form
where, p and q are integers. A rational number in decimal form as a terminating or repeating decimal.
An irrational number is a number that can't be expressed in the form
and have a non-repeating, non-terminating decimal.

Square root of 9 is 3 as 
Here,
is an irrational number as
is an irrational number. The number 6 is a rational number.
So, the sum of a rational number and irrational number is always an irrational number.
Therefore, the correct option is option D