Help identifying the correct statistical test.

A clinical trial was conducted in a sample of 5 obese adults to examine the effects of a diet program on BMI. Participants consumed the low carb diet at every meal for 6 months. To compare the difference in the BMI before and after the diet program, which statistical method should be used?

Kruskal-Wallis or Wilcoxon signed-rank, or Mann-Whitney, or Friedman test?

Please help me understand the rationale for the answer. Thank you.