Which term describes organisms that are unaffected by harmful substances in their environment?

The term "resistant" describes organisms that can withstand or are unaffected by harmful substances in their environment. This concept is commonly applied in various biological contexts, such as microbiology, where certain bacteria may develop resistance to antibiotics, allowing them to survive exposure that would be lethal to other strains. In a broader ecological context, resistance can relate to an organism's ability to thrive in adverse conditions or the presence of toxins and pollutants in their habitat.

The other terms listed convey different meanings. "Reactive" suggests an organism's tendency to respond to stimuli or changes in their environment, but does not imply immunity to harmful substances. "Vulnerable" refers to an organism's susceptibility to harm or adverse effects, indicating that they can be easily affected by harmful agents. "Susceptible" also denotes an organism's lack of resistance, making them prone to harmful effects when exposed to toxins or pathogens. Thus, "resistant" is the most accurate term for describing organisms that are unaffected by harmful substances.