In an experiment, a scientist compares the effect of adding acid rain to
samples taken from two different lakes. the scientist observes that ten drops
of acid rain added to water taken from lake minnetonka in southern
minnesota does not change the ph from 7.5, but ten drops of acid rain added
to water taken from upper kintla lake in glacier national park causes the ph
to change from 7.5 to 5.2.
which conclusion is supported by these observations?

Lake Minnetonka water is buffered instead of Upper Kintla Lake water that doesn´t have molecules to "muffle" the acid rain added.

Explanation:

When Adding Acid or a Base to a sample, and its pH doesn´t change, it will tell us that something is happening and there must be some molecules in the sample that are reacting with the Acid or Base added and that is why, the sample´s pH stay the same. These "molecules" muffle the Acid or Base, and they are called "buffer".

As you can see in the image below, there is an example with a typical Bicarbonate Buffer when adding HCl, the Bicarbonate molecules will react with the HCl, making it to change in other molecule, and losing its acid power.

When the Acid Rain was added to the Upper Kintla Lake sample, the pH changed instantly, because there were any molecule to react with the HCl added and HCl molecules remain the same, making the pH to drop from 7.5 to 5.2