Step 1: Concept
For the square root to be defined, the radicand must be non-negative: $x^2 + x - 2 \ge 0$.
Step 2: Factorization
$x^2 + 2x - x - 2 \ge 0 \implies (x + 2)(x - 1) \ge 0$.
Step 3: Interval Analysis
The roots are $-2$ and $1$. The inequality holds for $x \le -2$ or $x \ge 1$.
Final Answer: (D)