The least acidic compound, among the following is:

Solution

Chemistry ma’am · 4 steps

Find the acidic H in each one. Only D has no O–H at all, just a terminal alkyne C–H.

1. Locate the acidic H. Acidity depends on how stable the anion is after the H⁺ leaves. A, B and C each lose H⁺ from an O–H. D is an ester, HC≡C-COOC₂H₅, so its only acidic hydrogen is the one on the sp carbon of the alkyne.
2. Oxygen versus carbon. Oxygen is far more electronegative than carbon, so it holds a negative charge much better. In B and C the charge is also spread by resonance over several oxygens, and in A it spreads over oxygen and the ring.

   Compound	Acidic H	Negative charge ends up on	Approx. pKₐ
   B, C₆H₅SO₃H	-SO₃H	three equivalent O atoms	≈ -2.8
   C, butanoic acid	-COOH	two equivalent O atoms	≈ 4.8
   A, 3-ethoxyphenol	phenolic O–H	O, partly delocalised into the ring	≈ 9.6
   D, ethyl propiolate	≡C–H	one sp carbon	far above 10

3. Why D stays weak. The sp carbon has 50% s-character, and the ester group pulls some electron density away, so this C–H is more acidic than an ordinary C–H. A carbanion is still much less stable than any oxyanion, so D is the weakest acid of the four.
4. So. The order is B > C > A > D. The least acidic compound is D, option (b).

The idea to keep. To rank acids, look at the conjugate base. Charge on oxygen, spread by resonance, means a strong acid. Charge on a single carbon means a very weak acid, even an sp carbon.

Answer from the NTA final answer key. Solution by PrepWiser.