The question pertains to the fineness of cement, which is typically measured in terms of its specific surface area. Let's understand and solve the question based on the provided options:

The specific surface of cement refers to the total surface area of cement particles per unit mass. This is a critical property as it influences the rate of hydration and strength gain of cement.

Now, let's evaluate the options:

• Option 1: kg/cm² - This unit is generally used for pressure or stress measurement and is not applicable to measuring surface area.
• Option 2: cm²-g - This unit suggests surface area per gram. While similar to surface area measurement, the combination is unusual in the context of cement fineness, primarily used for different materials with different scale measurements.
• Option 3: m²/kg - This is the standard unit used to express the specific surface area of cement. Here, the surface area is measured in square meters, and the mass in kilograms, aptly describing the fineness.
• Option 4: m²-g - Like option 2, this option uses gram as the mass unit, which is uncommon for cement industry standards.

Based on the analysis, the correct answer is: m²/kg. This is because the specific surface area, indicating the fineness of cement, is best represented by the unit that considers surface per unit mass, making it aligned to standard industry practices.

Conclusion: The specific surface area, or fineness, of cement is expressed in m²/kg, as it effectively measures how much surface area of the cement is available for reaction per kilogram of cement.