Odpowiedź D, ponieważ 

A. $\sqrt{2}\cdot \sqrt{6}=\sqrt{2\cdot 6}=\sqrt{12}\ne 12$  

B. $\frac{\sqrt{60}}{\sqrt{5}}=\sqrt{\frac{60}{5}}=\sqrt{12}\ne 12$  

C. $2\sqrt{2}\cdot \sqrt{3}=\sqrt{4}\cdot \sqrt{2}\cdot \sqrt{3}=\sqrt{4\cdot 2}\cdot \sqrt{3}=\sqrt{8}\cdot \sqrt{3}=\sqrt{8\cdot 3}=\sqrt{24}\ne 12$  

D. $3\sqrt{2}\cdot \sqrt{8}=\sqrt{9}\cdot \sqrt{2}\cdot \sqrt{8}=\sqrt{9\cdot 2}\cdot \sqrt{8}=\sqrt{18}\cdot \sqrt{8}=\sqrt{18\cdot 8}=\sqrt{144}=12$