Which one of the following LD50's is most toxic?

• A. 145
• B. 1500
• C. 15
• D. 30

Answer: (C)

LD50 is the dose that will kill 50% of a test population, and it’s a standard way to compare acute toxicity. The smaller the LD50 value, the more toxic the substance is, because only a tiny amount is needed to be lethal under the same conditions and route of exposure.

In the given values, the smallest LD50 is 15, so that substance is the most toxic under equivalent testing conditions (same unit, same exposure route). The other numbers—30, 145, and 1500—are all larger, meaning more of those substances would be required to reach the same lethal effect, making them less toxic than the one with the 15 value. Remember to compare LD50 values using the same units and the same exposure route to keep the ranking accurate.