Astatement, or set of statements, that describes general principles about how variables relate to one another is a

theory                                                                                        

Explanation:

Theory: The term theory is defined as the process through which various aspects of a phenomenon is being defined. It is based on the process of defining various concepts and how one principle is related to that of the other. It generally explains the idea to solve specific problems.

In other words, it is defined as a phenomenon that explains scientifically acceptable principles and is followed by various methods.

hygrometers and hair thermometers (often found as part of hygrometers/psychrometers) can be used to measure humidity. barometers are used to measure atmospheric pressure, while anemometers are used to measure wind speed. so the 2nd and 4th choices should be the correct answers.