Pls solve this:

6. Domain the cos^-1 [2x^​2 -3] where [.] denotes greatest integer function, is. 


$$\begin{gathered} \left(A\right) \left[1, \sqrt{\frac{5}{2}}\right] \left(B\right) \left[- \sqrt{\frac{5}{2}},-1\right] \\ \left(C\right) \left[- \sqrt{\frac{5}{2}},-1\right]\cup \left[1, \sqrt{\frac{5}{2}}\right] \left(D\right) None of these. \end{gathered}$$