The correct answer is D. It asks about a cause-and-effect relationship between two variables.
Step-by-step explanation:
In most cases, the purpose of a scientific investigation is to understand the way one phenomenon is affected by a specific factor. For example, through scientific investigation, it is possible to know to what extent nutrition affects learning or how temperature changes affect plant growth. This implies the factors involved, which are formally called variables, have a cause-and-effect relationship because it is expected one of the factors affects or changes the other. Moreover, in this investigation researchers cannot modify or change the results. In this context, the most appropriate statement for a scientific investigation is "It asks about a cause-and-effect relationship between two variables."