Question

when the results are not statistically significant, you should
accept the null hypothesis.
accept the alternative hypothesis.
reject the null hypothesis.
fail to reject the null hypothesis.

Explanation:

In hypothesis testing, when results are not statistically significant, it means there is not enough evidence to reject the null hypothesis. We do not "accept" the null hypothesis as we can never be 100% certain it is true. We only fail to reject it based on the current evidence.

Answer:

fail to reject the null hypothesis.