If an element has three isotopes with known masses, what other information is needed to find the average atomic mass of the element

The abundance percentage of each isotope

You need to find the abundance. Then, multiply the abundance by 100, and add that to the mass for each isotope. Basically, for each isotope, take the percentage abundance and add it to the mass. Multiply each calculation of these together to get your average atomic mass,

I believe the answer would be:

A. Three groups of plants—a group fertilized by X, a group fertilized by Y, and a control group with no fertilizer.

I hope this helps.