Q.Which of the following is the least acidic compound ?

Step 1. Recall what governs phenol acidity (section 11.4.4, Problem 11.5). A ring substituent's effect on acid strength is governed by whether it STABILISES or DESTABILISES the phenoxide conjugate base by resonance/induction, relative to plain phenol. Electron-WITHDRAWING groups (like -NO2) stabilise the negative charge of the phenoxide (by resonance and induction), making the phenol MORE acidic than plain phenol. Electron-DONATING groups (like -CH3, an alkyl group, by its +I/hyperconjugative effect) instead DESTABILISE the phenoxide's negative charge, making the phenol LESS acidic than plain phenol. …

Comparing the electronic effect of each substituent (electron-donating -CH3 vs. electron-withdrawing -NO2) on phenoxide conjugate-base st …

• Assuming the exact ring position of the substituents must be known to answer this; the qualitative ranking (donor group weakens acidity, withdrawing group strengthens it, relative to phenol itself) holds regardless of exact position. …