Correct order of acidity of protons marked A-D:

Step 1: Identifying Acidic Sites

• $\ce{H_C}$: Carboxylic acid proton (Strongest acid, $pK_a \approx 4-5$).
• $\ce{H_D}$: Benzylic/Alpha proton (Stabilized by resonance with ring and carbonyl).
• $\ce{H_A}$: Terminal alkyne proton ($sp$ hybridized Carbon, $pK_a \approx 25$).
• $\ce{H_B}$: Benzylic/Propargylic proton ($sp^3$ Carbon, less acidic than $sp$).

Step 2: Acidic Strength Order $\ce{H_C > H_D > H_A > H_B}$

$\boxed{\text{Answer: (b)}}$