Which of the following is a valid hypothesis for a plant growth experiment with sunlight as the variable?

• A. If plants receive more sunlight, then they will grow taller than plants with less sunlight.
• B. If plants receive more water, they will grow taller than plants with less water.
• C. If sunlight increases, they will die quickly.
• D. If plants receive more fertilizer, height will not change.

Answer: (A)

A solid hypothesis for a plant growth study with sunlight as the variable states a clear cause-and-effect by linking the amount of light to a measurable growth outcome. The best choice says that more sunlight will lead to taller plants than less sunlight. This is a proper, testable expectation: it uses an if-then structure, compares two light conditions, and predicts a measurable result (height). If the experiment shows taller plants with more light, the hypothesis is supported; if not, it can be tested and revised.

The other ideas don’t fit as well. Shifting the variable to water means you’d be testing a different factor, not sunlight. Predicting that more sunlight will cause plants to die is possible but not a straightforward growth hypothesis and relies on extreme or uncontrolled conditions. Saying more fertilizer won’t change height focuses on a separate factor and expresses a no-change prediction not about how sunlight affects growth.