What type of test helps identify a safe concentration for organisms exposed to a toxic substance?

The correct choice is the range finding test. This type of test is specifically designed to establish safe concentration levels of toxic substances for organisms.

In a range-finding test, various concentrations of a toxic substance are exposed to the test organisms, and the effects are observed. The goal is to determine the highest concentration at which no harmful effects are noticeable, which helps establish a baseline for safe exposure levels. This type of testing is crucial for understanding the threshold of toxicity and informs further testing protocols, ensuring that subsequent tests are conducted within a safe and relevant concentration range.

Other options serve different purposes in toxicity testing. For instance, a static test measures toxicity without any water renewal, which might not yield the full understanding of the substance’s effects over time. A definitive test aims to determine the exact lethal concentration or dose, but it does not focus on establishing safe levels and often follows range-finding tests. A screening test is typically quicker and less detailed, used to identify potential hazards but not optimized for finding safe concentration limits.