Which of the following acts as an acid in an aqueous solution?

Which one of the following substances has the highest proton affinity?

In the reaction $HNO_3 + HF \to H_2NO_3^+ + F^-$,which species acts as a base?

According to the $Br\text{\o}nsted$ theory,which are the acids in the following reaction?
$NH_3 + H_2O \rightleftharpoons NH_4^+ + OH^-$

Weak acids are partially ionised in aqueous solutions. The ionisation constants of some acids are given below. Arrange these acids in increasing order of their acid strength.

Acid Index	Acid Name and Ionisation Constant $(K_a)$
$1$	Formic acid $(HCOOH)$: $1.8 \times 10^{-4}$
$2$	Hypochlorous acid $(HClO)$: $3.0 \times 10^{-8}$
$3$	Nitrous acid $(HNO_2)$: $4.5 \times 10^{-4}$
$4$	Hydrocyanic acid $(HCN)$: $4.9 \times 10^{-10}$

What is the increasing order of basic strength for $Cl^{-}, CH_3COO^{-}, OH^{-},$ and $F^{-}$?