Question

I'll say B is the correct answer. One key concept to chemical equilibrium is that the concentration of the products in relative to the reactants (or reagents) does not change. The reaction is still going on, but there's no NET change. so a would be wrong because any observable characteristics would hint a change in the composition, so no, not in equilibrium. As for d, at equilibrium, even though there's still reaction, there's no visible changes. And c just doesn't make sense to me ( I'll be happy if anybody can explain to me what it means).