Flow is considered laminar when:

Laminar flow is characterized by smooth and orderly movement of fluid particles where each layer of fluid slides past the adjacent layer with minimal mixing or disruption. In this context, the correct choice highlights that in laminar flow, individual particle paths do not cross. This indicates that the fluid moves in parallel layers, allowing for predictable and stable flow patterns.

When the paths of fluid particles do not intersect, it means that each particle is following a specific trajectory without interference from others, which is a fundamental characteristic of laminar flow. This type of flow generally occurs at low velocities and in fluids with higher viscosity, where frictional forces between layers dominate over inertial forces.

In contrast, other choices describe scenarios that do not pertain to laminar flow. Irregular curves and chaotic motion suggest a turbulent state, where fluid particles are mixing and creating complex flow patterns. Straight path lines, while they might seem acceptable in a laminar scenario, do not capture the essence of distinct particle pathways that clearly do not cross, making the definition of laminar flow most accurately reflected in the correct answer.