Assertion (A): The mean of first 9 natural numbers is 4.5.

Statement 2: Mean = $\dfrac{\text{Sum of all observations}}{\text{Total number of observations}}$

1. (A) is true, (R) is false.

2. (A) is false, (R) is true.

3. Both (A) and (R) are true, and (R) is the correct reason for (A).

4. Both (A) and (R) are true, and (R) is the incorrect reason for (A).

First 9 natural numbers = 1, 2, 3, 4, 5, 6, 7, 8, 9

By formula,

$\Rightarrow \text{Mean } = \dfrac{\text{Sum of all observations}}{\text{Total number of observations}}\\[1em] = \dfrac{1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9}{9}\\[1em] = \dfrac{45}{9}\\[1em] = 5.$

∴ (A) is false, (R) is true.

Hence, option 2 is the correct option.