Step 1: Understanding the Concept
The geometry depends on the hybridisation of the central metal ion, which is decided by the ligand field strength.
Step 2: Calculation
In \([\text{CoF}_6]^{3-}\), \(x+6(-1)=-3\), so cobalt is \(\text{Co}^{3+}\) with \(3d^{6}\).
\(\text{F}^-\) is a weak field ligand, so no pairing of \(3d\) electrons occurs.
Step 3: Hybridisation
The outer \(4s\), three \(4p\) and two \(4d\) orbitals are used, giving \(sp^{3}d^{2}\) hybridisation (high spin).
Six ligands with \(sp^{3}d^{2}\) hybridisation sit at the corners of an octahedron.
Final Answer:
The complex is octahedral.
\[ \boxed{\text{(C) }\text{Octahedral}} \]