**Core Concept:** Secular trend refers to the gradual, long-term changes in the prevalence or incidence of diseases in a population over time. These changes may be due to various factors, including advances in public health measures, improvements in living conditions, and changes in lifestyle.

**Why the Correct Answer is Right:** The correct answer, **D. Regression Analysis**, is used to identify the relationship between the changes in a disease's prevalence or incidence and the changes in the population's characteristics or exposures. By plotting these changes over time, regression analysis can help determine if the relationship is significant and whether it is consistent with a secular trend.

**Why Each Wrong Option is Incorrect:**

A. **Time Series Analysis:** While Time Series Analysis can also analyze data over time, it is primarily used for studying the fluctuations in a single variable over a period, without considering the relationship with other variables or trends.

B. **Descriptive Statistics:** Descriptive statistics provide summary measures of data but do not help in identifying the underlying relationships or trends between variables.

C. **Cross-Sectional Studies:** Cross-sectional studies provide information on the prevalence of a disease or condition at a given point in time and cannot detect trends over time.

**Clinical Pearl:** It is essential to choose the appropriate statistical analysis method based on the research question and the nature of data available. Regression analysis, when applied correctly, can help identify secular trends and inform public health interventions to address the changing health needs of a population.