Correct Option: 4
Statement 4, "Some colours are not pleasant," is a particular negative (O type) proposition. Statement 1, "All colours are pleasant," is a universal affirmative (A type) proposition. In classical logic, A and O propositions are contradictories. Contradictory propositions cannot both be true; if one is true, the other must be false. Given that Statement 4 is true, its contradictory, Statement 1, must therefore be false.
Incorrect Options:
Regarding option 1 ("1 and 2 are true"): As established, Statement 1 is false. Consequently, this option, which claims Statement 1 is true, cannot be correct.
Regarding option 2 ("3 is true"): Statement 3, "No colour is pleasant," is a universal negative (E type) proposition. If Statement 4 ("Some colours are not pleasant") is true, it implies the existence of at least one colour that lacks pleasantness. However, this does not logically necessitate that no colour is pleasant. It is entirely possible for some colours to be unpleasant while other colours are pleasant. Therefore, Statement 3 cannot be definitively concluded as true.
Regarding option 3 ("2 is false"): Statement 2, "Some colours are pleasant," is a particular affirmative (I type) proposition. The truth of Statement 4 ("Some colours are not pleasant") indicates that at least one colour is not pleasant. This does not preclude the possibility that some other colours are pleasant. Both "Some colours are not pleasant" and "Some colours are pleasant" can be simultaneously true within a given set of colours. Hence, Statement 2 cannot be definitively concluded as false.