Final answer:
The independent variable in Monica's report "The effect the amount of rain has on the clarity of well water" is the amount of rain, which she manipulates to observe its effects on well water clarity.
Step-by-step explanation:
In Monica's investigation titled "The effect the amount of rain has on the clarity of well water," the independent variable is the amount of rain. An independent variable is the factor that is changed by the researcher to observe how it affects a dependent variable. The purpose of Monica's experiment is likely to determine how variations in rainfall influence the visual transparency or the presence of particulates in well water, which is the dependent variable in this case. In Gary's experiment, if he wanted to test the effect of the amount of water on plant growth, the independent variable would be the same as Monica's: the amount of water. Gary's hypothesis might also involve water quality, but in this context, the question specifically mentions the quantity of water. Therefore, b. 'the amount of plant food' and other options do not describe the independent variable in Monica's report.