Answer:
Water.
Explanation:
When hydrogen gas and oxygen gas combine, it will form water. Its chemical reaction can be written as :
[tex]H_2+O_2\rightarrow H_2O[/tex]
Now balancing the above equation,
[tex]2H_2+O_2\rightarrow 2H_2O[/tex]
So, if hydrogen gas and oxygen gas combine it will form water. Hence, the correct option is (D).