Answer:
The correct answer is option c. "growing cuttings from the same Hydrangea macrophylla plant under controlled conditions that vary only with regard to soil pH".
Step-by-step explanation:
A experiment that would prove that flower color in Hydrangea macrophylla plants is determined primarily by soil conditions is growing cuttings from the same Hydrangea macrophylla plant under controlled conditions that vary only with regard to soil pH. If the cuttings of the same plant (which means that they have the same genotype) develop different flower colors, it will prove that this trait is determined mainly by the soil pH.