The least acidic compound, among the following is:

Step 1: Comparing Acidic Strength Acidity depends on the stability of the conjugate base.

• Phenol: Resonace stabilized phenoxide ion.
• Sulfonic Acid ($\ce{R-SO3H}$): Highly stabilized by resonance with three oxygens.
• Carboxylic Acid ($\ce{R-COOH}$): Stabilized by resonance with two oxygens.
• Terminal Alkyne ($\ce{R-C \equiv C-H}$): The negative charge on $sp$-hybridized carbon is less stable than resonance-stabilized oxygens.

Step 2: Identifying the Least Acidic Terminal alkynes ($pK_a \approx 25$) are significantly less acidic than carboxylic acids ($pK_a \approx 5$), phenols ($pK_a \approx 10$), or sulfonic acids ($pK_a < 0$).

$\boxed{\text{Answer: (a) Compound D }}$