**Step 1 — Check the Reason (R).**
$$1 + 2 + 3 + \dots + n = \frac{n(n+1)}{2}$$
This is the standard result. $\therefore$ **Reason (R) is TRUE.**

**Step 2 — Check the Assertion (A).**
$$\text{Mean} = \frac{\text{Sum of observations}}{\text{Number of observations}} = \frac{\dfrac{n(n+1)}{2}}{n} = \frac{n+1}{2}$$

The Assertion claims $\dfrac{n-1}{2}$, which is wrong.

$\therefore$ **Assertion (A) is FALSE.**

**Step 3 — Choose the option.**
Assertion false, Reason true.

**Answer : (D)**