Answer : The acid strength of HCl is strongest among the given series. So, the strength decreases as per this trend HCl > [tex] HSO_{4} ^{-} [/tex] > HCN;
So when identifying the weakest base it has to be CN as it has less acid strength as compared to the series as it acts as a lewis base which donates its pair of electrons. They are weak bases generally but when compared with acids they are considered to be the strongest.
