Thanks!!Nice question. Now some people may confuse this graph with that of rate of reaction against concentration of substrate (which is identical in shape to the one in the question!). According to the data in the question, we have absolutely no idea what the enzyme concentrations and substrate conc are, and we don't know which factor is limiting the rate of the reaction.
So we have NO IDEA as to whether 'most enzymes have free active sites' or not. Maybe all active sites are saturated. Maybe there are may enzymes free, without any substrate bound to them. Who knows? That's why A isn't a very great option to choose.
B is correct, because if you look at the graph, you'll see that the time X is somewhere near the beginning of the reaction (time 0). So very less product was made at time X, and thus a large amount of substrate is left (yet to react).
Hope this helps!