hi
the bigger the Ka, the stronger the acid
acid strength = HF > HNO2 > HCN
the stronger the acid, the weaker the conjugate base
conjugate base strength = F⁻ < NO2⁻ < CN⁻
we can eliminate option B) C) and D).
Cl⁻ is the weakest base because it has no effect on pH when dissolved in a aqueous solution
H3O⁺ (SA = Strong Acid) gives H2O (WCB = Weak conjugate Base)
HF (WA) gives F⁻ (SCB)
so base strength F⁻ > H2O and the correct solution is A)
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