A wire can be broken by 400 kg.wt. The load required to break the wire of double the thickness of the same material will be - 
 

We know that,

$Breaking stress=\frac{F}{A}$

$F\propto r^2$ Because breaking stress is same,

$$\begin{gathered} \frac{F_2}{F_1}={\left(\frac{r_2}{r_1}\right)}^2 \\ \frac{F_2}{400}={\left(\frac{2r}{r}\right)}^2 \\ {F}_2=1600kgwt \end{gathered}$$

Hence the correct answer is 1600 kg wt.