What is the correct equation for the K constant for the chemical equation 2NO(g) + O₂(g) ⇌ 2NO₂(g)?

Options:
a) [NO₂]²/[NO]²[O₂]
b) [NO₂]/[NO]²[O₂]
c) [NO₂]²/[NO][O₂]
d) [NO₂]/[NO][O₂]