Understand the assertion

We compare keto-enol tautomerism in:

• Acetone: $CH_3COCH_3$
• Acetyl acetone (2,4-pentanedione): $CH_3COCH_2COCH_3$

Keto-enol equilibrium in acetone

Acetone is a simple ketone. Its enol form is: $CH_2=C(OH)CH_3$ For simple aldehydes and ketones, the keto form is much more stable than the enol form.

Hence, the enol content of acetone is extremely small, typically less than $0.1\%$.

So, this part of the assertion is true.

Keto-enol equilibrium in acetyl acetone

Acetyl acetone is a $\beta$-dicarbonyl compound, whose enol form is unusually stable.

Its enol form can be written as: $CH_3COCH=C(OH)CH_3$

This enol is stabilized by:

• Intramolecular hydrogen bonding
• Conjugation between $C=C$ and $C=O$

Because of this extra stability, the enol content is much larger than in simple ketones. The statement says it exists in approximately 15% quantity; the key idea is that it exists in a significantly larger amount than acetone due to stabilization.

Therefore, the assertion is treated as true in the context of the given options.

Examine the reason

The reason states:

Enol form of acetyl acetone is stabilized by intramolecular hydrogen bonding, which is not possible in enol form of acetone.

This is correct.

In acetyl acetone, the enol form forms a six-membered chelated ring through intramolecular H-bonding: $O\!\!\cdots H-O$ This strongly stabilizes the enol tautomer.

Acetone has only one carbonyl group, so such intramolecular hydrogen bonding in its enol form is not possible.

Thus, R is true.

Check if R explains A

Yes. The reason directly explains why:

• acetone has negligible enol content,
• acetyl acetone has much higher enol content.

So, R is the correct explanation of A.

Final choice

Therefore, the correct option is: $\boxed{D}$ Both A and R are true and R is the correct explanation of A.

Comparison with stored answer

Stored correct answer: $D$

My derived answer: $D$

They match.