The answer is (b) Milk
(b) Milk:
– Milk exhibits the Tyndall effect due to its colloidal nature. It contains dispersed particles, such as fat globules and proteins, which are large enough to scatter light, making the beam visible.

Now, for the others:
(a) Salt solution:
– Salt solutions typically do not show the Tyndall effect. The dissolved ions in the solution are too small to scatter light significantly.
(c) Copper sulfate solution:
– Similar to salt solutions, copper sulfate solutions primarily consist of dissolved ions and do not have large enough particles to display the Tyndall effect.
(d) Starch solution:
– Starch solutions, being composed of smaller molecules, do not exhibit the Tyndall effect. The particles in the solution are not large enough to scatter light noticeably.