Information gathered by a scientist abut the toxicity of chemical X and chemical Y showed that they had individual safe limits for fish at particular concentrations. But when they were used together at the safe concentrations, there were extensive fish kills. This is an example of .

Synergism

Explanation:

Synergism -

It refers to the process of interaction between the any two or more chemicals , i.e. , combining the chemicals increases the effect of the chemical , is referred to as synergism .

The individual drug or chemical might not be strong or harmful in any respect , but combining it with various other drugs or chemical , might make it very strong or harmful .

Hence ,from the given scenario of the question ,

The correct answer is synergism .