Answer
The correct option is D.
Explanation with reason
According to balance chemical equation
A. 3H2 + N2 → NH3
in the above reaction there are 6 mole of hydrogen atom and 2 mole of nitrogen atom on reactant side, one mole of nitrogen atom and 3 mole of hydrogen atom on product side. This is not a balance chemical equation.
B. H2 + N2 → 2NH3
There are 2 mole of hydrogen atom and 2 mole of nitrogen atom on reactant side and 2 mole of nitrogen and 6 mole of hydrogen atom on product side. This is not a balance chemical equation.
C. H2 + N2 → NH3
There are 2 mole of hydrogen as well as nitrogen atom on reactant side and i mole of nitrogen and 3 mole of hydrogen atom on product side. so, this is also not a balance chemical equation.
D. 3H2 + N2 → 2NH3
There are 6 mole of hydrogen atom and 2 mole of nitrogen atom on both side. So this is a balance chemical equation.
Conclusion
It is concluded from above discussion that D equation correctly represents the chemical reaction that form ammonia.