Although benzene is highly unsaturated it does not undergo addition reactions. The explanation of this can be suggested as

Options

A. `pi`-electrons of benzene ring are delocalised
B. since `pi`-electrons are present inside the ring, addition cannot take place
C. cyclic structures do not show addition reactions
D. benzene is not a reactive compound.

Explanation

To understand why benzene, despite being highly unsaturated, does not undergo addition reactions, we can break down the explanation into several steps:

### Step 1: Understanding Benzene Structure
Benzene (C₆H₆) is a cyclic compound consisting of six carbon atoms arranged in a ring, with alternating single and double bonds. However, this representation is simplified.

**Hint:** Remember that benzene has a unique structure that differs from simple alkenes.

### Step 2: Delocalization of Pi Electrons
In benzene, the pi electrons from the double bonds are not localized between two specific carbon atoms. Instead, they are delocalized over the entire ring structure. This means that the electrons are spread out and shared among all six carbon atoms.

**Hint:** Think about how delocalization affects the stability of the molecule.

### Step 3: Resonance Structures
Benzene can be represented by several resonance structures, where the position of the double bonds can shift. This resonance leads to a more stable structure because the energy is lowered due to the delocalization of electrons.

**Hint:** Consider how resonance contributes to the stability of benzene compared to other unsaturated compounds.

### Step 4: Nature of Addition Reactions
Addition reactions typically occur in compounds with localized pi electrons. In these reactions, the pi bond is broken, and new atoms or groups are added to the molecule. However, because the pi electrons in benzene are delocalized, breaking them to add new atoms would disrupt the stable resonance structure.

**Hint:** Recall that breaking bonds in a stable system is energetically unfavorable.

### Step 5: Conclusion
Due to the delocalization of pi electrons and the stability provided by resonance, benzene does not undergo addition reactions. Instead, it typically participates in substitution reactions, where one atom or group is replaced by another without disrupting the overall stability of the aromatic system.

**Hint:** Focus on the difference between addition and substitution reactions in terms of stability and electron distribution.

### Final Answer
The correct explanation for why benzene does not undergo addition reactions is that the pi electrons of benzene are delocalized, which stabilizes the molecule and prevents the addition of new atoms or groups. Therefore, the answer is option A: "The pi electrons of benzene are delocalized."