Explanation:

The sex ratio is a fundamental demographic statistic used to understand the balance between males and females in a population. Its calculation method can vary by country, but in India, there is a standard definition. \nOption A, 'Female population / Male population $\times 100$', is incorrect. While it uses the correct numerator and denominator, the multiplier of $100$ is not the standard for India. This formula would yield the number of females per $100$ males. \nOption B, 'Female population / Male population $\times 1000$', is the correct answer. In India, the sex ratio is traditionally defined as the number of females per $1000$ males. This formula accurately reflects that definition, providing a standardized measure for comparison across different regions and time periods. \nOption C, 'Male population / Female population $\times 100$', is incorrect. This formula calculates the number of males per $100$ females, which is the inverse of the standard Indian definition and uses an incorrect multiplier. \nOption D, 'Male population / Female population $\times 1000$', is incorrect. This formula calculates the number of males per $1000$ females, which is the inverse of the standard Indian definition for sex ratio. While some countries might use this inverse definition, it is not the standard practice in India.