Final answer:
A one-tailed t-test is the suitable test for determining if the mean body temperature of healthy adults is less than 37 degrees Celsius, used when the population variance is unknown and the sample size is small.
Step-by-step explanation:
The most suitable test for determining if the mean body temperature reading for the population of healthy adults is less than 37 degrees Celsius is a one-tailed t-test. This type of test is chosen because the population standard deviation is unknown and the sample size is relatively small (less than 30), hence the sample standard deviation is used to estimate the population standard deviation. Additionally, the hypothesis is directional, specifying that the mean body temperature is less than a certain value, which warrants a one-tailed test. To perform the hypothesis test correctly, it is assumed that the data comes from a population that is approximately normally distributed and that the data are from a simple random sample. If these assumptions are met, even with an unknown population variance and a sample size smaller than 30, a Student's t-distribution is applicable.