Which property explains why the path of light is not visible when a beam passes through a true solution?

The correct answer is Particles are smaller than one nanometre.

Step-by-Step Explanation:

1. Understanding the Tyndall Effect:
The scattering of a beam of light as it passes through a medium containing suspended particles is known as the Tyndall effect. When light is scattered by these particles, the path of the light beam becomes visible.

2. Condition for Light Scattering:
For light to scatter significantly and make its path visible, the size of the solute or colloidal particles must be comparable to or larger than the wavelength of visible light (typically around 400 nm to 700 nm).

3. Particle Size in a True Solution:
In a true solution (such as salt dissolved in water), the solute particles exist as individual molecules or ions. The diameter of these particles is extremely small—less than 1 nanometre ($<1\text{nm}$ or ${10}^{-9}\text{m}$).

4. Conclusion:
Because the solute particles in a true solution are smaller than 1 nanometre in diameter, they are far too tiny to scatter visible light rays. As a result, light passes straight through without scattering, rendering the path of the light beam invisible.