**Core Concept**
Correlation between two variables is a statistical measure used to describe the relationship between them. It helps in understanding whether there is a linear or non-linear relationship between the variables, and the strength of the relationship. In clinical research, correlation analysis is often used to identify potential associations between different variables, such as risk factors and disease outcomes.

**Why the Correct Answer is Right**
The correct answer is a scatter plot, which is a graphical representation of the relationship between two variables. In a scatter plot, each data point represents a single observation, with the x-axis representing one variable and the y-axis representing the other variable. The points on the plot are scattered across the graph, and the pattern of the scatter can indicate a positive, negative, or no correlation between the variables. Scatter plots are particularly useful in identifying outliers and non-linear relationships between variables.

**Why Each Wrong Option is Incorrect**
**Option A:** is incorrect because a bar chart is used to compare categorical data across different groups, not to show the relationship between two continuous variables.
**Option B:** is incorrect because a histogram is used to display the distribution of a single variable, not to show the relationship between two variables.
**Option C:** is incorrect because a box plot is used to compare the distribution of a variable across different groups, not to show the relationship between two variables.

**Clinical Pearl / High-Yield Fact**
When interpreting scatter plots, it's essential to look for patterns, such as a positive or negative slope, and to consider the strength of the correlation using measures like the correlation coefficient (r). A strong correlation does not necessarily imply causation, and additional analysis is often needed to understand the underlying mechanisms.

**Correct Answer:** C. Scatter plot.