Low speed, subsonic and
incompressible as given in the question means that the density remains constant
throughout the venturi.
It does not matter whether the cross
sectional area increases or decreases.
Temperature will however alter the
density, with an increase in temperature reducing it, resulting in a reduced
mass flow, and vice versa, but since the flow is incompressible the density
will still remain constant throughout the venturi irrespective of its value.
Notably in accordance with the law of
continuity the mass flow is the product of the density, velocity and cross
sectional area.
If density remains constant, then the
velocity will increase as the cross sectional area decreases and vice versa if
the law of constant mass or law of continuity is to remain true.