Interpret the assertion

Assertion A states:

• $\mathrm{p}K_a$ of phenol $= 10.0$
• $\mathrm{p}K_a$ of ethanol $= 15.9$

These are standard approximate values and are correct.

So, Assertion A is true.

Use the meaning of $\mathrm{p}K_a$

Recall:

• Lower $\mathrm{p}K_a$ means stronger acid.
• Higher $\mathrm{p}K_a$ means weaker acid.

Since $\mathrm{p}K_a(\text{phenol}) = 10.0 < \mathrm{p}K_a(\text{ethanol}) = 15.9$ phenol is more acidic than ethanol.

Check the reason

Reason R says: Ethanol is stronger acid than phenol.

This is opposite to the true order.

Therefore, Reason R is false.

Why phenol is more acidic than ethanol

On deprotonation:

• Phenol gives phenoxide ion, which is stabilized by resonance.
• Ethanol gives ethoxide ion, which has no resonance stabilization.

Greater stability of the conjugate base means greater acidity.

Hence, phenol is more acidic than ethanol.

Choose the correct option

• Assertion A: True
• Reason R: False

Therefore, the correct option is: D: $A$ is true but $R$ is false.