Examine the following statements: 1. All colours are pleasant. 2. Some colours are pleasant. 3. No colour is pleasant. 4. Some colours are not pleasant. Given that statement 4 is true, what can be definitely concluded?

The question asks us to deduce what can be definitely concluded if statement 4 is true.

Let's analyze the statements:

1. All colours are pleasant. (Universal Affirmative - A)
2. Some colours are pleasant. (Particular Affirmative - I)
3. No colour is pleasant. (Universal Negative - E)
4. Some colours are not pleasant. (Particular Negative - O)

These are standard categorical propositions. The key relationship here is contradiction.

• Statement 1 (A) and Statement 4 (O) are contradictory propositions. This means if one is true, the other must be false, and vice-versa.
• Statement 3 (E) and Statement 2 (I) are contradictory propositions. This means if one is true, the other must be false, and vice-versa.

Given: Statement 4 "Some colours are not pleasant" is true.

Now let's evaluate the options:

A) 1 and 2 are true Since statement 4 (O) is true, its contradictory, statement 1 (A), must be false. Therefore, option A cannot be correct as it claims 1 is true.

B) 3 is true If statement 4 (O) "Some colours are not pleasant" is true, it does not necessarily mean statement 3 (E) "No colour is pleasant" is true. For example, if some colours are pleasant and some are not, then O is true but E is false. So, 3 is not definitely true.

C) 2 is false If statement 4 (O) "Some colours are not pleasant" is true, it does not necessarily mean statement 2 (I) "Some colours are pleasant" is false. Both can be true (e.g., if out of 10 colours, 5 are pleasant and 5 are not). So, 2 is not definitely false.

D) 1 is false Statement 1 "All colours are pleasant" (A) and Statement 4 "Some colours are not pleasant" (O) are contradictory. Since statement 4 is given as true, its contradictory, statement 1, must be false. This is a definite conclusion.

The final answer is $\boxed{D}$