Identify the ligands in the complex

The complex ion is: $[Th(C_2O_4)_4(H_2O)_2]^{4-}$

Ligands present:

• $4$ oxalato ligands, $C_2O_4^{2-}$
• $2$ aqua ligands, $H_2O$

Determine denticity of each ligand

• Oxalato $(C_2O_4^{2-})$ is a bidentate ligand. So, each oxalato ligand contributes $2$ donor atoms.
• Water $(H_2O)$ is a monodentate ligand. So, each water molecule contributes $1$ donor atom.

Calculate total coordination number

From $4$ oxalato ligands: $4 \times 2 = 8$

From $2$ water ligands: $2 \times 1 = 2$

Therefore, total coordination number of Th is: $8 + 2 = 10$

Match with the options

The correct option is: $\boxed{10}$

So, Option B is correct.