If A + B = C, then tanAtanBtanC = ?

Answer: Option D

A + B = C
tan(A + B) = tanC
$$\frac{\tan \text{A}+\tan \text{B}}{1-\tan \text{A}\tan \text{B}}=\tan \text{C}$$
tanA + tanB = tanC - tanAtanBtanC
tanAtanBtanC = tanC - tanA - tanB