At equilibrium the concentration of reactants and product become:

Explanation

Equilibrium occurs when the rate of forward reaction becomes equal to the rate of backward reaction, thus making the concentration of reactants and products constant hence 'option C' is correct. 'Option A' is very wrong because concentrations of reactants and products do not become zero at equilibrium. It can be understood by common sense that if the concentrations were to become zero, then the reaction will not occur. 'Option B' is the most commonly chosen incorrect answer. Equilibrium is frequently misunderstood as equal concentrations of reactants and products, which is extremely wrong! 'Option D' is wrong because concentrations of reactants and products are limited (not infinite).