When we have the concentration of a compound, let's say HF, what we actually have represented in that number is the concentration of both elements, so if we have 1.0*10^-5 M of HF, this means that we also have 1.0*10^05 M of H and 1.0*10^-5 M of F, since there is only 1 mol (or element) of each one, the concentration will be the same for both. Therefore the concentration for H+ or H3O+ Hydronium ion will be 1.0*10^-5, letter A
