Final answer:
A statement that predicts the outcome of a scientific study and describes the relationship between variables is called a hypothesis. It is a testable tentative explanation and, when tested, may lead to the development of a theory.
Step-by-step explanation:
The statement which predicts the outcome of a scientific study by describing the relationship between variables of interest is known as a hypothesis. A hypothesis is a tentative explanation that can be tested by further investigation or through a controlled experiment. If a hypothesis is rejected by the experiment, this does not mean the experiment was a failure, rather, it's often a success because it provides data that helps refine scientific understanding. A theory, on the other hand, is a well-supported explanation of observations. Thus, among the given options (a Hypothesis, b Theory, c Observation, d Experiment), the correct answer is a) Hypothesis.