Which expression verifies that a unit vector u = (a,b,c) has magnitude 1?

Which expression verifies that a unit vector u = (a,b,c) has magnitude 1?

• A. |u| = sqrt(a^2+b^2+c^2)
• B. |u| = a^2+b^2+c^2
• C. |u| = a+b+c
• D. |u| = 1

Answer: (A)