1. Understand Assertion A

   Assertion: Benzene is more stable than hypothetical cyclohexatriene.

   This is true.

   • If benzene were just a simple cyclohexatriene with three isolated double bonds, it should behave like a normal polyene.
   • But benzene shows extra stability due to resonance/aromaticity.
   • Its six $\pi$ electrons are delocalised over the entire ring.
   • This delocalisation lowers the overall energy of the molecule, making benzene more stable than a hypothetical localized structure of cyclohexatriene.

2. Understand Reason R

   Reason: The delocalised $\pi$ electron cloud is attracted more strongly by nuclei of carbon atoms.

   This is also true.

   • In benzene, the $\pi$ electrons are spread over all six carbon atoms.
   • Because of delocalisation, the electron cloud is distributed symmetrically around the ring.
   • This delocalised electron cloud experiences effective attraction from all the carbon nuclei, leading to stabilization.

3. Check whether R explains A

   Yes, it does.

   • Benzene’s greater stability arises from delocalisation of $\pi$ electrons.
   • The delocalised electron cloud is stabilized by attraction to the carbon nuclei over the whole ring.
   • Therefore, Reason R correctly explains why benzene is more stable than hypothetical cyclohexatriene.

4. Evaluate options

   • A: A is true but R is false $\rightarrow$ Incorrect
   • B: Both A and R are correct and R is the correct explanation of A $\rightarrow$ Correct
   • C: Both A and R are correct but R is NOT the correct explanation of A $\rightarrow$ Incorrect
   • D: A is false but R is true $\rightarrow$ Incorrect

Final Answer

Option B is correct.