Delta iron, also known as δ-iron or ferrite, is a solid phase of iron that exists at elevated temperatures. It undergoes a phase transformation from austenite to delta ferrite during cooling. The correct answer to the question is option 'C', which states that delta iron occurs between 910°C and 1539°C. Let's understand why this is the correct answer.

Delta iron formation temperature:
Delta iron forms when the temperature of iron is in the range of 910°C to 1539°C. This temperature range is above the melting point of iron, which is around 1538°C. At temperatures above the melting point, iron is in the liquid phase. However, during the cooling process, as the temperature decreases, the liquid iron undergoes a phase transformation and solidifies into delta iron.

Phase transformation from austenite to delta ferrite:
At high temperatures, iron exists in the austenite phase, which has a face-centered cubic (fcc) crystal structure. As the temperature decreases, the crystal structure of iron changes. At temperatures above 910°C, the austenite phase transforms into delta ferrite, which has a body-centered cubic (bcc) crystal structure. This transformation is known as the ferrite transformation.

Key points:
- Delta iron is a solid phase of iron.
- It forms between 910°C and 1539°C.
- It occurs during the cooling process of liquid iron.
- Delta iron has a bcc crystal structure.
- It is formed from the transformation of austenite to delta ferrite.

In conclusion, delta iron occurs at temperatures between 910°C and 1539°C, which is above the melting point of iron. It forms during the cooling process as the austenite phase transforms into delta ferrite.