Concept:
Demography is the statistical study of human populations. It focuses on population structure, composition, and changes over time.
Explanation:
Demographic characteristics typically include:
Population size,
Age distribution,
Birth and death rates,
Sex ratio.
Sex ratio represents the number of females per 1000 males and is a key demographic indicator.
Per capita income relates to economic conditions, not demography.
Crime rate is a social indicator.
Social security refers to welfare and policy measures.
Conclusion:
Among the given options, sex ratio directly deals with the demography of a town.
Final Answer: \(\boxed{\text{Sex ratio}}\)