Chọn B
Ta có \(cos^{2} x+cosx=0\Leftrightarrow \left[\begin{array}{l} {cosx=0} \\ {cosx=-1} \end{array}\right. \Leftrightarrow \left[\begin{array}{l} {x=\frac{\pi }{2} +k\pi } \\ {x=\pi +k2\pi } \end{array}\right. {\rm \; \; }(k\in {\rm Z}).\)
Vì với \(k\in {\rm Z}: \frac{\pi }{2} <x<\frac{3\pi }{2} \Rightarrow \left[\begin{array}{l} {\frac{\pi }{2} <\frac{\pi }{2} +k\pi <\frac{3\pi }{2} } \\ {\frac{\pi }{2} <\pi +k2\pi <\frac{3\pi }{2} } \end{array}\right. \Rightarrow k=0\Rightarrow x=\pi .\)