When testing a hypothesis about a single mean, if the sample size is 20, the population standard deviation is unknown, and the population is assumed to be a normal distribution, the correct test statistic to use is

T-Test

Explanation:

A t-test is a form of inferential statistics used to assess if there is a substantial difference between the two-group means that can be connected to certain characteristics.

It's often used whenever the data-sets would follow a normal distribution and may have unknown deviations, such as the data set recorded as the result of flipping a coin 100 times. A t-test has been used as a testing tool for hypothesis, which allows the testing of a population-specific assumption.

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