Hi there! Based on the question you provided, it seems like Marley's mom's company is trying to test the effectiveness of its new sunscreen products. To answer your question, they might want to investigate all of the test variables mentioned except for the numerical strength of the sunscreen (option D). This is because the strength of the sunscreen is already known and should not be changed during the testing process.
Option A, resulting sunburns, would be a good variable to investigate as it measures the effectiveness of the sunscreen in preventing sunburns. Option B, frequency of application, would also be important to investigate as it can determine how often the sunscreen needs to be applied for optimal protection. Option C, length of exposure to the sun, would also be a significant variable to investigate as it can determine how long the sunscreen can protect the skin from sun damage.
I hope that helps! Let me know if you have any otherquestions.