B

Step 1: Understand that electromagnetic induction occurs when a changing magnetic field induces an electromotive force (e.m.f.) in a conductor.
Step 2: Lenz's law states that the direction of the induced e.m.f. will always be such that it opposes the change that produced it.
Step 3: This opposition is a direct result of the conservation of energy and ensures that the induced current creates a magnetic field that counteracts the original changing magnetic field.
Step 4: Therefore, the correct answer is Lenz's law, as it specifically addresses the direction of the induced e.m.f.
Therefore, the answer is B.