Which of the following will have the highest $pK_a$ value;

The correct answer is Option (3) → 

Reason (step-by-step):

• Highest pKₐ = weakest acid

• Acid strength depends on how stable the conjugate base (RCOO⁻) is.

Now compare the given compounds:

1. Benzoic acid (C₆H₅COOH)
   – Phenyl ring shows a –I effect, stabilising the conjugate base
   – Hence more acidic → lower pKₐ

2. Phenylacetic acid (C₆H₅CH₂COOH)
   – Phenyl group still exerts a –I effect, though weaker than benzoic acid
   – Still more acidic than acetic acid

3. Acetic acid (CH₃COOH) 
   – CH₃ group has +I effect, which destabilises the conjugate base
   – Hence weakest acid → highest pKₐ

4. Chloro-substituted acid (Cl–CH₂–CH₂–COOH)
   – Cl has strong –I effect, greatly stabilising conjugate base
   – Therefore most acidic → lowest pKₐ

Final order of acidity (strong → weak):

Chloro acid > Benzoic acid > Phenylacetic acid > Acetic acid

Thus, the compound with the highest pKₐ is:

 Acetic acid (Option 3)