Answer:
b. 2
There are necessary 2 hydrogen molecules to produce 2 water molecules.
Step-by-step explanation:
The balanced equation is already given:

So, 2 molecules of diatomic hydrogen react with 1 molecule of diatomic oxygen to obtain 2 water molecules.
As you can see in the attached image, the 2 molecules of diatomic hydrogen have 2 hydrogen atoms each (i.e. 4 hydrogen atoms), the molecule of diatomic oxygen has 2 oxygen atoms and the 2 water molecules have 2 hydrogen atoms and 1 oxygen atom each (i.e. 4 hydrogen atoms and 2 oxygen atoms)