The question requires a good knowledge of the meaning of Tyndall effect or scattering of light by false solutions in chemistry
The student observed a beam of visible light through mixture A because A exhibits Tyndall effect
Tincture of Iodine does not show Tyndall effect because its solution consists of particles less than 1 nanometer in solution.
A solution may be classified as a true solution or a false solution. A true solution consists of particles which are less than 1 nanometer in solution. A true solution does not scatter visible light.
A false solution, on the other hand consists of particles larger than 1 nanometer and does scatter visible light.
Starch in water is an example of a false solution or colloid hence it exhibits Tyndall effect. The scattering of visible light by a solution is known as Tyndall effect.
Tincture of iodine and sodium chloride in water are all true solutions hence they do not scatter visible light.
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