The observation that could be used as evidence that the amount of space in an environment can affect an animal's growth is:
"Two lizards of the same species grow different sizes in different-sized terrariums."
This observation suggests that the varying sizes of the terrariums may influence the growth of the lizards, providing a direct link between the amount of space available and the growth outcomes observed. The other options involve factors other than space (like food supply or species differences) that may also affect growth, but they do not isolate space as a variable.