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Acidity determines how much acid is present in the given compound. An acid is a chemical compound that gives off a hydrogen ion or a proton in the solution.

The acidity of different compounds can be compared by comparing various factors such as the hetero-atoms present, resonance, and hybridization.

For example, the presence of an electron-withdrawing group or atom, presence of resonance and hybridization such that double and triple bonds are present in the structure increases the acidity of the compound

a.

Two of the three given compounds have highly electronegative atoms, acting as electron-withdrawing groups. As is stated above, the presence of an electron-withdrawing group increases the acidity of the compound. Hence, the order of increasing acidity is:

Order of increasing acidity in a.

b.

The acidity increases down the column. So, the compounds containing –Cl and –SH groups are more acidic. Among the groups containing –Cl and –SH, acidity increases across a row. Hence, the order of increasing acidity is:

Order of increasing acidity in b.

c.

The two compounds containing a highly electronegative atom, i.e., the oxygen atom in their functional group, are more acidic than those with no functional groups at all.

One of the two compounds containing the –OH group includes an electron-withdrawing atom, i.e., chlorine, further increasing its acidity. Hence, the order of increasing acidity is:

Order of increasing acidity in c.

d.

The given compounds will be compared based on their hybridization. As it is well known that the order of increasing electronegativity is as ${\text{sp}}^{\text{3}}{\text{<sp}}^{\text{2}}\text{<sp}$.

This is because the s-character increases in the given order; as the s-character increases, the electron-attracting ability increases, and the compound becomes more electronegative.

Hence, the order of increasing acidity is:

Order of increasing acidity in d.