Final answer:
Van Helmont would have taken measurements of the mass of plants and soil, concluding that plant growth was attributed to water intake, as he found the soil mass unchanged while the plant's mass increased significantly.
Step-by-step explanation:
The measurements that Van Helmont would have taken are B) Mass of plants and soil. Van Helmont conducted experiments in the early 17th century to understand plant growth. He specifically measured the mass of a willow tree and the mass of the soil in which it was planted at the beginning and end of a five-year period, to determine the source of the increase in the plant's mass. His experiment concluded that plants grow by taking in water, which was a revolutionary idea at the time. This conclusion was drawn from the observation that the soil mass remained virtually unchanged while the plant gained significant mass.