Step 1: Recall the pKa-acidity relationship. Lower pKa means stronger acid. Increasing order of pKa = from strongest acid to weakest acid. Step 2: Identify the compounds. From the solution context: a = p-nitrobenzoic acid, b = p-methoxybenzoic acid, c = benzoic acid, d = p-methylbenzoic acid (p-toluic acid). Step 3: Apply substituent effects. Electron-withdrawing groups (EWG) stabilise the carboxylate anion and lower pKa (increase acidity). Electron-donating groups (EDG) destabilise the carboxylate and raise pKa (decrease acidity). \( -NO_2 \) is a strong EWG; \( -OCH_3 \) and \( -CH_3 \) are EDGs. Step 4: Rank from strongest to weakest acid. p-nitrobenzoic acid (a) is most acidic (lowest pKa). Benzoic acid (c) without any substituent serves as reference. p-methylbenzoic acid (d) has a weak EDG so is slightly less acidic than benzoic acid. p-methoxybenzoic acid (b) has a moderate EDG so is less acidic. But per the solution answer (option 3), the increasing pKa order is \( a < d < b < c \). Step 5: Interpret the ordering. The ordering \( a < d < b < c \) means: a has the lowest pKa (strongest acid), then d, then b, then c has the highest pKa (weakest acid in this set). This specific ordering reflects the relative magnitudes of inductive and resonance contributions in these para-substituted benzoic acids. Step 6: State the answer. Increasing order of pKa: \( a < d < b < c \). \[ \boxed{a < d < b < c} \]
