Step 1: Concept
Fleming's Left-Hand Rule is a mnemonic used to determine the direction of the force experienced by a current-carrying conductor placed in a magnetic field.
Step 2: Meaning
It establishes the relationship between three mutually perpendicular vectors: the magnetic field, the electric current, and the resulting physical force (motion).
Step 3: Analysis
When the thumb, forefinger, and middle finger of the left hand are held at right angles to each other: the forefinger represents the Magnetic Field, the middle finger represents the Current, and the thumb points in the direction of the Force/Motion.
Step 4: Conclusion
Thus, it describes the effect of a magnetic field acting on an electric current.
Final Answer: (D)