Resonance occurs due to

Options

A. delocalisation of lone pair of electrons
B. delocalisation of `sigma`-electrons
C. delocalisation of `pi`-electrons
D. oscillation of a proton.

Explanation

To answer the question "Resonance occurs due to," we need to understand the concept of resonance in chemistry. Here’s a step-by-step breakdown of the solution:

### Step 1: Define Resonance
Resonance is a phenomenon in chemistry where a molecule can be represented by two or more valid Lewis structures (resonance structures) that differ only in the arrangement of electrons. This occurs when there are multiple ways to arrange the electrons in a molecule.

**Hint:** Remember that resonance structures are not real; they are a way to represent the delocalization of electrons.

### Step 2: Identify the Causes of Resonance
Resonance occurs due to the delocalization of electrons, which can happen in two primary ways:
1. **Delocalization of Lone Pairs of Electrons:** This involves lone pairs of electrons interacting with pi bonds or other bonds in the molecule.
2. **Delocalization of Pi Electrons:** This occurs when pi bonds between adjacent atoms can shift, allowing for different arrangements of the double bonds.

**Hint:** Think about how electrons can move between different atoms or bonds in a molecule.

### Step 3: Analyze the Options Given
Now, let's analyze the options provided:
- **Delocalization of lone pairs of electrons:** This is a true statement as lone pairs can participate in resonance.
- **Delocalization of sigma electrons:** This is incorrect because sigma bonds are localized and do not participate in resonance.
- **Delocalization of pi electrons:** This is also true as pi electrons can shift between bonds.
- **Oscillation of a proton:** This is not related to resonance and is incorrect.

**Hint:** Focus on how electrons can move and interact in different ways to form resonance structures.

### Step 4: Conclusion
Based on the analysis, resonance occurs due to:
- The delocalization of lone pairs of electrons.
- The delocalization of pi electrons.

Thus, the correct answer to the question is that resonance occurs due to the delocalization of both lone pairs and pi electrons.

**Final Answer:** Resonance occurs due to the delocalization of lone pairs of electrons and the delocalization of pi electrons.