**Core Concept**
The question is testing the concept of correlation analysis in statistics, specifically the type of correlation that measures linear association between two continuous variables in the same individuals.

**Why the Correct Answer is Right**
Pearson's correlation coefficient (r) is a statistical measure that quantifies the linear relationship between two continuous variables. It ranges from -1 to 1, where 1 and -1 indicate perfect positive and negative linear relationships, respectively, and 0 indicates no linear relationship. The formula for Pearson's correlation coefficient involves the covariance of the two variables and their standard deviations. It is widely used in various fields of medicine, including research and clinical decision-making.

**Why Each Wrong Option is Incorrect**
* **Option A:** is incorrect because Spearman's rank correlation coefficient is used to measure the association between two ordinal or continuous variables, not linear association between two continuous variables.
* **Option B:** is incorrect because Cronbach's alpha is a measure of internal consistency or reliability of a set of items, not a correlation coefficient.
* **Option C:** is incorrect because Point Biserial correlation coefficient is used to measure the association between a continuous variable and a binary variable, not two continuous variables.

**Clinical Pearl / High-Yield Fact**
Pearson's correlation coefficient is sensitive to outliers and non-normality of the data, so it is essential to check the assumptions before interpreting the results.

**Correct Answer: C. Pearson's correlation coefficient (r).**