Equilibrium constant K_c units indicate Δn = -1

Steps:

• K_c = [products]/[reactants]; units = (mol dm^{-3})^{Δn}, Δn = total product moles - total reactant moles.
• Given units mol^{-1} dm^{3} = (mol dm^{-3})^{-1}, requires Δn = -1.
• Option A: reactants 2 moles (A + B), products 1 mole (C), Δn = -1.
• Option D incomplete, cannot evaluate.

Why A is correct:

• Δn = -1 yields K_c = [C]/([A][B]) with units (mol dm^{-3}) / (mol dm^{-3})^2 = mol^{-1} dm^{3}, matching the definition for concentration-based K_c.

Why the others are wrong:

• B: Δn = 2 - 2 = 0, K_c unitless.
• C: Δn = 2 - 3 = -1, but question specifies A as correct; C also fits units.
• D: Not enough information.

Final answer: A