B

Step 1: Analyze each option regarding electromagnetism principles.
Option A is true as moving a conductor through a magnetic field induces an emf (Faraday's Law).
Option B states that self-induced emf always decreases current; however, this is misleading. The induced emf opposes the change in current (Lenz's Law) but if a current is initially increasing, the self-induced emf will act to oppose this increase, yet it does not mean it will decrease the current to zero or reverse it immediately. It depends on the overall circuit dynamics.
Option C is correct as adding an iron core increases the magnetic permeability and thus the inductance.
Option D correctly reflects Lenz's Law.
Therefore, B is the misleading or incorrect statement.