In the S.I. system used in engineering mechanics, identify the correct unit for mass density (mass per unit volume).

Introduction / Context:
Density is a foundational property in mechanics and fluid mechanics. It links mass to the space it occupies and appears in nearly every conservation equation (mass, momentum, and energy). Correctly recalling the S.I. unit for density ensures dimensional consistency in formulas and correct interpretation of physical quantities across solids, liquids, and gases.

Given Data / Assumptions:

• Density (symbol rho) is defined as mass per unit volume.
• S.I. base unit of mass is kilogram (kg); S.I. unit of length is metre (m); volume is m^3.
• The question asks for the S.I. unit, not the symbol.

Concept / Approach:
By definition, density = mass / volume. Therefore, the unit must combine the mass unit (kg) with the inverse of volume (1/m^3). Any unit involving force (N), time (s), or area (m^2) would contradict the definition. Dimensional analysis is the most robust way to verify this quickly during exams.

Step-by-Step Solution:

Verification / Alternative check:

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Final Answer: