Step 1: Concept
For a square root function, the expression inside must be $\ge 0$.
Step 2: Meaning
$x^{2}+2x-15 \ge 0 \implies (x+5)(x-3) \ge 0$.
Step 3: Analysis
The inequality holds when $x \le -5$ or $x \ge 3$.
Step 4: Conclusion
The domain is $(-\infty,-5] \cup [3,\infty)$.
Final Answer: (D)