The correct answer is: C. size and shape of the cross section in a particular length remain constant.
A uniform flow is a flow in which the velocity of the fluid is the same at all points in a given cross section and at all times. This means that the size and shape of the cross section in a particular length remain constant.
A steady flow is a flow in which the velocity of the fluid at a given point does not change with time. This does not mean that the velocity of the fluid is the same at all points in the flow.
A streamline flow is a flow in which the fluid particles move in smooth, parallel lines. This does not mean that the flow is uniform.
A non-uniform flow is a flow in which the velocity of the fluid is not the same at all points in a given cross section or at all times. This means that the size and shape of the cross section in a particular length change.