Step 1: Concept
The norm (length) of a vector $v = (x, y, z)$ is $||v|| = \sqrt{x^2 + y^2 + z^2}$.
Step 2: Meaning
First, find the sum vector $\alpha + \beta = (-1+2, 0+0, 1-2) = (1, 0, -1)$.
Step 3: Analysis
Now calculate the norm of $(1, 0, -1)$: $\sqrt{1^2 + 0^2 + (-1)^2} = \sqrt{1 + 0 + 1}$.
Step 4: Conclusion
This equals $\sqrt{2}$.
Final Answer: (D)