Which statement represents a testable hypothesis about a pond ecosystem?

A testable hypothesis is a statement that can be supported or refuted through experimentation or observation. The selected statement about frogs jumping into the water when startled is a clear example of this. It presents a cause-and-effect relationship that can be observed in a controlled setting. By startling frogs and observing their behavior, one can collect data to determine whether or not they jump into the water, allowing for a definitive conclusion to be drawn.

In contrast, the other options provide different types of information. The statement about frogs representing a population describes a concept in ecology without proposing any testable outcomes. The assertion that water is an abiotic component describes a fact about ecosystems but does not lend itself to experimentation. Finally, the statement about fish being happier if given more food introduces a subjective measure of happiness, which is difficult to quantify or test scientifically. Thus, option D stands out as the only statement that can directly lead to a scientific investigation.