$\begin{array}{1 1} \text{(a) the concentration of the un-ionized molecules must be relatively small compared with the ion concentrations } \\
\text{(b) the concentration of the un-ionized molecules must be relatively small compared with the ion concentrations } \\
\text{(c) the substance ionizes to a large degree } \\
\text{(d) the concentration of the un-ionized molecules must be relatively large compared with the ion concentrations } \end{array} $