What is the effect that occurs between lone pair and π-bond?

The correct option is Resonance.

Let's understand the interaction between a lone pair and a π-bond step-by-step:
1. Conjugation: When a lone pair of electrons on an atom is separated from a double bond (π-bond) by exactly one single bond (σ-bond), the system is said to be conjugated. For example, in vinyl chloride (CH2=CH-Cl), the lone pairs on the chlorine atom are conjugated with the carbon-carbon double bond.
2. Delocalization: Due to the overlap of the p-orbitals containing the lone pair and the π-system, the lone pair of electrons is not localized on a single atom. Instead, these electrons can flow toward the π-bond, while the π-electrons shift to the adjacent carbon atom.
3. Resonance: This shift and delocalization of π-electrons and lone pairs is represented by drawing multiple contributing structures (resonance structures). The actual molecule exists as a hybrid of these structures. Therefore, the interaction that occurs between a lone pair and a π-bond is called the resonance effect (or mesomeric effect).

Let's briefly examine why the other options do not describe this effect:
• Inductive Effect: This is a permanent polarization that travels through σ-bonds due to differences in electronegativity, rather than the delocalization of lone pairs and π-electrons.
• Electromeric Effect: This is a temporary effect involving the complete transfer of a shared pair of π-electrons to one of the bonded atoms, but it occurs only in the presence of an attacking reagent.
• Hyperconjugation: This involves the delocalization of σ-electrons (typically from C-H bonds) into adjacent empty p-orbitals or π-systems, not the interaction of a lone pair.