The volumes of 4N HCl and 10N HCl required to make 1L of 6 N HCl are

$$\begin{gathered} {\left(\mathrm{VN}\right)}_{\mathrm{mix}}={\mathrm{V}}_1{\mathrm{N}}_1+{\mathrm{V}}_2{\mathrm{N}}_2 \\ 1\times 6=4{\mathrm{V}}_1+10{\mathrm{V}}_2 \end{gathered}$$

From options, 

 $$\begin{gathered} \left(\mathrm{A}\right) \left(4\times 0.75\right)+(0.25\times 10)=5.5 ; \mathrm{not} \mathrm{suitable} \\ \left(\mathrm{B}\right) (4\times 0.25)+(0.75\times 10)=8.5 ; \mathrm{not} \mathrm{suitable} \\ \left(\mathrm{C}\right) (4\times 0.67)+(0.33\times 10)=5.98\approx 6 \end{gathered}$$

Hence option(C) is correct answer.