Hydrangea macrophylla is a species of plant with blue or pink flowers. Flower color in Hydrangea macrophylla plants is thought to be determined primarily by soil conditions rather than by inherited differences. Which of the following will provide the most direct evidence that flower color in Hydrangea macrophylla is due primarily to soil conditions?

Phenotypic expression can be influenced by the environment. The question implies that soil conditions may cause these changes. One of the main abiotic factors is soil pH. Studying the color in response to soil pH is a great way to gauge whether or not the flower color is determined by soil conditions.

DNA sequence comparisons and genetic crosses are unlikely to provide information about how the environment affects flower color. They will only show genetic influences of flower color.

Flower fossils would have long decomposed to the point where color is difficult to gauge.