**Core Concept**
When comparing mean values between two independent population groups, it is essential to use a statistical test that accounts for the independence of the samples. This is because the samples are not paired or matched in any way, and the comparison is between two distinct groups.

**Why the Correct Answer is Right**
The unpaired t-test, also known as the independent samples t-test, is used to compare the means of two independent groups. This test is suitable when comparing the mean hemoglobin (Hb) values between two population groups, as it takes into account the variability within each group and the difference between the groups. The unpaired t-test calculates the t-statistic, which indicates whether the difference between the means is statistically significant.

**Why Each Wrong Option is Incorrect**
**Option A:** The paired t-test is used to compare the means of two related or paired groups, such as before and after treatment in the same individual. This is not applicable when comparing two independent population groups.
**Option C:** The Chi-square test is a non-parametric test used to compare categorical data between two or more groups. It is not used to compare mean values between groups.
**Option D:** The Fisher test, also known as the Fisher exact test, is a non-parametric test used to compare categorical data between two groups. It is not used to compare mean values between groups.

**Clinical Pearl / High-Yield Fact**
When comparing mean values between two groups, it is essential to check for normal distribution of the data and equal variances between the groups. If the data is not normally distributed or has unequal variances, non-parametric tests such as the Mann-Whitney U test may be more appropriate.

✓ Correct Answer: B. Unpaired t test.