Pick out the statement which is not relevant in the discussion of colloids:
A.Sodium aluminium silicate is used in the softening of hard water
B.Potash alum is used in shaving rounds and as a styptic in medicine
C.Artificial rain is caused by throwing electrified sand on the clouds from an aeroplane
D.Deltas are formed at a place when the river pours its water in to the sea

Hint: We also know that the colloids contain ultramicroscopic particles or large molecules of non – crystalline and homogeneous substances which are dispersed to another substance. Gels, sols, and emulsions are examples of colloids. And in colloids, the small and tiny particles will shake out to another particle in a large volume. Fog is an example of colloid.

Complete answer:
Among these options, the Sodium aluminium silicate used in the softening of hard water is not relevant in the discussion of colloids. Because, the sodium aluminum silicate is used in the case of hard water for the softening purposes. And this will not be related to colloids.
Here it takes place by a chemical substitution of calcium salt to the zeolite and there is a formation of calcium zeolite as precipitate and due to the formation of this precipitate, the hard water will become soft. Hence, option (A) is correct.
Potash alum is used in shaving rounds and as a styptic in medicine is relevant in the discussion of colloids. Because it is related to colloids. Hence, option (B) is correct.
Artificial rain is caused by throwing electrified sand on the clouds from an aero plane, which is happening on the basis of the property of colloids. Therefore, it is relevant in the discussion of colloids. Hence, option (C) is incorrect.
Deltas are formed at a place when the river pours its water into the sea on the basis of colloidal properties and it is very important in the discussion of colloids. Hence, the option (D) is incorrect.

Note:
In this case, only the statement, “Sodium aluminium silicate is used in the softening of hard water” is not relevant in the discussion of colloids. Because, here the sodium silicate is used to reduce the hardness of water and there is a formation of respective precipitate. But there is no relationship between this colloid and the hardness of water or formation of precipitate.