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The CONSERVATION OF MASS equation for an incompressible fluid ($\rho$ = const) is:
A{'text': '$\\nabla \\cdot \\mathbf{u} = 0$ (divergence-free velocity field)', 'label': 'A'}
B{'text': '$\\rho \\mathbf{u} = 0$', 'label': 'B'}
C{'text': '$\\nabla \\rho = 0$', 'label': 'C'}
D{'text': '$\\partial u/\\partial t = 0$', 'label': 'D'}
Answer & Solution
Correct answer: A. {'text': '$\\nabla \\cdot \\mathbf{u} = 0$ (divergence-free velocity field)', 'label': 'A'}
General mass conservation: $\partial \rho/\partial t + \nabla \cdot (\rho \mathbf{u}) = 0$. For incompressible flow ρ = const: $\nabla \cdot \mathbf{u} = 0$. The velocity field has zero divergence — fluid cannot accumulate.
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