Given the equation for a system at equilibrium:
N2(g) + 3H2(g) <-> 2NH3(g) + energy

If only the concentration of N2(g) is increased, the concentration of

1. NH3(g) increases
2. NH3(g) remains the same
3. H2(g) increases
4. H2(g) remains the same

1. increases.

Explanation:

Any change in the equilibrium is studied on the basis of Le-Chatelier's principle.This principle states that if there is any change in the variables of the reaction, the equilibrium will shift in the direction to minimize the effect.

For the given equation:

If the concentration of is increased , according to the Le-Chatlier's principle, the reaction has to shift to right or forward direction. In order to do that  the concentration of products has to increase.

Thus the concentration of increases.