Which one is known as a fusion mixture?

The correct option is Na₂CO₃ + K₂CO₃.

A fusion mixture is a mixture of two salts that has a lower melting point than either of the individual components. This is known as a eutectic mixture. Specifically, a mixture of anhydrous sodium carbonate (Na₂CO₃) and potassium carbonate (K₂CO₃) combined in an equimolar ratio (1:1 molar ratio) is commonly referred to as a fusion mixture in analytical chemistry.

Sodium carbonate has a melting point of approximately 851 °C, and potassium carbonate has a melting point of approximately 891 °C. When mixed together in an equimolar ratio, the mixture forms a eutectic system with a significantly lower melting point of about 710 °C.

This lower melting point makes the mixture highly effective as a flux in solid-state fusion reactions. It is widely used to convert insoluble compounds, such as silicates and sulfates, into soluble carbonates or oxides, facilitating their subsequent chemical analysis.