Answer: t-statistic.

The correct answer is **A (t-statistic)**. For a hypothesis test concerning the mean difference between two normally distributed populations with unknown variances, the appropriate test statistic is the **t-statistic**. This is because the test involves paired observations, and the t-statistic is calculated as: \[ t = \frac{d - \mu_{d0}}{s_d / \sqrt{n}} \] where: - \( d \) is the sample mean difference, - \( \mu_{d0} \) is the hypothesized mean difference (often zero), - \( s_d \) is the standard error of the mean difference, - \( n \) is the number of paired observations. The t-statistic follows a t-distribution with \( n - 1 \) degrees of freedom. **Option B (F-statistic)** is incorrect because the F-test is used for comparing variances, not mean differences. **Option C (Chi-square statistic)** is also incorrect, as it is used for tests concerning the variance of a single normally distributed population, not mean differences between two populations.

Author: LeetQuiz Editorial Team