Which theory assumes that all magnetic substances are composed of tiny molecular magnets?

Weber's Theory is the correct answer because it specifically addresses the behavior of magnetic substances and posits that all such substances are made up of tiny magnets at the molecular level. This perspective explains how external magnetic fields can influence these small magnetic domains, leading to the observable magnetic properties of materials.

In contrast, Einstein's Theory primarily deals with the theory of relativity, which focuses on the relationship between space, time, and gravity, and does not pertain to magnetism. Ohm's Law is fundamental in understanding electrical circuits, detailing the relationship between voltage, current, and resistance, but does not involve magnetic properties. Faraday's Law deals with electromagnetic induction, explaining how a changing magnetic field can induce an electric current in a circuit, which is a different concept from the fundamental magnetic properties of materials. Thus, Weber's Theory uniquely addresses the question about the composition of magnetic substances at a molecular level.