C : N ratio of normal soil is

The Carbon-to-Nitrogen ($\text{C}:\text{N}$) ratio measures the relative proportion of organic carbon to total nitrogen present in soil. For an average, fertile, well-cultivated surface soil, the stabilized $\text{C}:\text{N}$ ratio typically ranges between $10:1$ and $12:1$. Therefore, 10 : 1 is the best fit option.