Step 1: Identifying the elements.
The elements from Boron to Oxygen have the following trend in IE$_1$:
Boron (B) → 1086.5, Carbon (C) → 800.6, Nitrogen (N) → 1402.3, Oxygen (O) → 1313.9
Step 2: Analyzing the trends.
The ionization energies for Nitrogen (N) are higher than those for Carbon (C) and Oxygen (O) due to the stability of its half-filled p-orbital configuration.
Step 3: Conclusion.
The value of IE$_1$ for Nitrogen is 1402.3 kJ/mol, corresponding to option (3).