Denoting L as liquid and M as solid in a phase-diagram with the subscripts representing
different phases, a eutectoid reaction is described by

The correct option is: M₁ → M₂ + M₃

Step-by-step Explanation:

To understand why this is the correct description of a eutectoid reaction, let us define the terms and phases involved in phase diagrams:
1. L represents a liquid phase.
2. M (with subscripts such as M₁, M₂, M₃) represents different solid phases.
3. The arrow (→) indicates the direction of the phase transformation during cooling.

A eutectoid reaction is an isothermal, reversible reaction in which one solid phase (upon cooling) transforms into two distinct, different solid phases simultaneously.

Let us analyze the given options in detail:
• M₁ → M₂ + M₃: A single solid phase (M₁) transforms into two different solid phases (M₂ and M₃). This matches the definition of a eutectoid reaction.
• L₁ → M₁ + M₂: A single liquid phase (L₁) transforms into two different solid phases (M₁ and M₂). This describes a eutectic reaction.
• L₁ + M₁ → M₂: A liquid phase (L₁) reacts with a solid phase (M₁) to form a single, new solid phase (M₂). This describes a peritectic reaction.
• M₁ + M₂ → M₃: Two solid phases react to form a new single solid phase. This describes a peritectoid reaction.

Therefore, the reaction represented by M₁ → M₂ + M₃ is the correct description of a eutectoid reaction.