In the given chemical reaction:
\[ 2H_2 + O_2 \rightarrow 2H_2O \]
Before the reaction, there are 4 atoms of hydrogen, as indicated by the 2 molecules of \( H_2 \) (each containing 2 hydrogen atoms).
After the reaction, the products are \( 2H_2O \). Each water molecule (\( H_2O \)) contains 2 hydrogen atoms, so in 2 molecules of \( H_2O \), there are:
\[ 2 \text{ molecules } \times 2 \text{ H atoms/molecule} = 4 \text{ H atoms} \]
Thus, the number of hydrogen atoms present after the reaction is 4.
So the correct response is:
4