Final answer:
In this case, conducting an experiment is the best design to determine the effect of a new fertilizer on tomato plants.
Explanation:
Experiment: The best design for the biologist to determine the effect of a new fertilizer on tomato plants is through experiment. They can conduct a controlled experiment by planting groups of tomato plants with different fertilizers to observe and measure the impact on plant growth.
Independent Variable: Brand of fertilizer.
Dependent Variable: Number of tomatoes.
Learn more about Experiment design for testing fertilizer effect on plants here:
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