The population of interest to Kristen while investigating students' opinions at her high school on the new school hours proposed by the school board is the "All students at her high school."
This is because she wants to know the opinion of the students at high school, and the best way to know is to ask the students themselves.
The other options are wrong because the research is not about them but her school's students' opinions.
Hence, in this case, it is concluded that the correct answer is "All students at her high school."
The available options to the question are the following:
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