$100\;g$ of substance has $7\;g$ of nitrogen.
$\therefore \; xg$ of substance has $\large\frac{7}{100} \times $$xg$ of nitrogen
For minimum molecular weight, it must have at least one N atom.
$\therefore \large\frac{7}{100} $$\times x =14$
$x= \large\frac {14 \times 100}{7}$$=200$
Hence b is the correct answer.