If 2*3 = 13 and 3 * 4 = 5, then the value of 5 * 12 is ?

Answer: Option D

$$\begin{array}{rl}& \text{Clearly, }a\ast b=\sqrt{a^2+b^2}\\ & \therefore 5\ast 12\\ & =\sqrt{5^2+{12}^2}\\ & =\sqrt{25+144}\\ & =\sqrt{169}\\ & =13\end{array}$$