Answer:
[tex]\boxed{\text{c. The concentration of reactants is much greater than that of products.}}[/tex]
Explanation:
NO₂ + NO₃ ⇌ N₂O₅; K = 2.1 × 10⁻²⁰
We often write K as
[tex]K = \dfrac{[\text{Products}]}{[\text{Reactants}]}[/tex]
If K is large, more of the molecules exist as products.
If K is small, more of the molecules exist as reactants.
[tex]\text{Since K is small, }\\\boxed{\textbf{the concentration of reactants is much greater than that of products.}}[/tex]
a. b., and d. are wrong. The concentration of reactants is greater.