What is the value of ( - 17 3 )?

Answer: Option C

$$\begin{array}{rl}& \cos \left(-\frac{17\pi }{3}\right)\\ & =\cos \left(-\frac{17\times 180}{3}\right)\\ & =\cos \left(-1020\right)\\ & =\cos \left[3\times 360-60\right]\\ & =\cos 60\\ & =\frac{1}{2}\end{array}$$