When conducting a plant-response investigation, what should be done to ensure the validity of the results?

The correct option is Keep all other conditions constant except for the variable being tested. This answer is correct because it matches the approved learning objective to apply safe and valid practical design to plant-response investigations, including control variables, repeats, light direction and gravity treatment in the subtopic Control and coordination.

The correct option is Keep all other conditions constant except for the variable being tested. Keep all other conditions constant except for the variable being tested is correct because it directly supports the approved learning objective to apply safe and valid practical design to plant-response investigations, including control variables, repeats, light direction and gravity treatment. This belongs to the subtopic Control and coordination within Plant hormones (biology only), so the explanation must stay tied to that curriculum context. The other options are incorrect because they either do not answer this learning objective, use a vague statement, or move away from Control and coordination.