**Core Concept**
Demography is the scientific study of the size, structure, and distribution of populations, including their growth, mortality, fertility, and other demographic characteristics. It is a multidisciplinary field that combines statistics, sociology, anthropology, and economics to understand population trends and dynamics.

**Why the Correct Answer is Right**
Morbidity refers to the incidence or prevalence of disease within a population. While morbidity is an important aspect of public health, it is not directly considered a demographic variable. Demography focuses on the characteristics of populations, such as age, sex, and marital status, rather than the health status of individuals. **Option C** is incorrect because morbidity is more closely related to epidemiology, which is the study of the distribution and determinants of disease.

**Why Each Wrong Option is Incorrect**
**Option A:** Mortality, which refers to the rate of death within a population, is a key demographic variable that is closely related to life expectancy and population growth.
**Option B:** Fertility, which encompasses birth rates and reproductive patterns, is a fundamental demographic characteristic that influences population growth and structure.
**Option D:** Marriage, which is a social institution that affects family formation and population structure, is also a demographic variable that is relevant to understanding population trends.

**Clinical Pearl / High-Yield Fact**
Demography is essential for planning and implementing public health interventions, as it helps policymakers understand population needs and trends. By analyzing demographic data, healthcare professionals can identify areas with high disease burdens, predict future healthcare needs, and develop targeted interventions to improve population health.

**✓ Correct Answer: C. Morbidity**