Final answer:
The researchers would likely test the effect of carbon dioxide on the growth rate and urushiol production of poison ivy, due to the association between increased CO2 levels and more robust plant growth and oil synthesis.
Step-by-step explanation:
The hypothesis that researchers might test regarding poison ivy growing in an environment with increased carbon dioxide levels is: a) The effect of carbon dioxide on poison ivy growth rate and oil production. This is because the information provided states that increased levels of carbon dioxide can cause poison ivy to grow larger, faster, and produce more urushiol, the oil that causes skin rashes. Recognizing poison ivy is essential for avoiding allergic reactions, as its leaves grow in groups of three, leading to the adage, 'leaves of three, leave it be.' The relationship between rising atmospheric carbon dioxide and changes in plant behavior is of great interest to researchers, especially as it pertains to weeds and allergenic plants like poison ivy.