Simplify the following expression: \( \sqrt{15} \cdot 3 \sqrt{6} \) \( \begin{array}{ll}\text { A) } 90 & \text { C) } 3 \sqrt{21} \\ \text { B) } 3 \sqrt{10} & \text { D) } 9 \sqrt{10}\end{array} \)

To simplify the expression \( \sqrt{15} \cdot 3 \sqrt{6} \), you can start by rearranging the terms: \[ = 3 \cdot \sqrt{15 \cdot 6} \] Now, calculate \( 15 \cdot 6 \): \[ 15 \cdot 6 = 90 \] So the expression becomes: \[ 3 \cdot \sqrt{90} \] Next, simplify \( \sqrt{90} \): \[ \sqrt{90} = \sqrt{9 \cdot 10} = \sqrt{9} \cdot \sqrt{10} = 3\sqrt{10} \] Now replace \( \sqrt{90} \) in the expression: \[ = 3 \cdot 3 \sqrt{10} = 9 \sqrt{10} \] Thus, the simplified expression is: \[ \boxed{9 \sqrt{10}} \] So the answer is option D) \( 9 \sqrt{10} \).