(a) Milk is a colloidal substance. Particulate matter present in the milk makes the light scatter which results in Tyndall effect. Because of Tyndall effect the milk got illuminated.
(b) Salt is a homogenous solution. Small particles present in salt solution do not scatter light rays hence there will be no Tyndall effect. Since salt solution did not exhibit Tyndall effect light is not illuminated.
(c) Detergent solution and sulphur solution exhibit the Tyndall effect.