The problem asks us to simplify the expression:

$\sqrt{15} \cdot \sqrt{15}.$

Solution:

1. Recall the property of square roots: $\sqrt{a} \cdot \sqrt{a} = a.$
2. Applying this property: $\sqrt{15} \cdot \sqrt{15} = 15.$

Thus, the simplified result is 15.

The correct answer is D.

Would you like a detailed explanation of square root properties or more examples?

Related Questions:

1. What happens when you multiply square roots of different numbers, e.g., $\sqrt{5} \cdot \sqrt{3}$?
2. How would you simplify $\sqrt{16} \cdot \sqrt{16}$?
3. Why does $\sqrt{a} \cdot \sqrt{a} = a$?
4. How would this change if the square roots involved fractions, e.g., $\sqrt{\frac{1}{4}} \cdot \sqrt{\frac{1}{4}}$?
5. How do you simplify a square root expression like $\sqrt{30}$?

Tip:

Always check if a square root multiplication forms a perfect square — this simplifies directly to the base number!