The correct option is **B. exerted on a moving charge at the point**. ### Detailed Explanation: 1. **Understanding Magnetic Fields**: - A magnetic field is a region around a magnetic material or a moving electric charge within which the force of magnetism acts. It is represented by magnetic field lines that indicate the direction and strength of the field. 2. **Force on Charges**: - The fundamental principle that defines the existence of a magnetic field is the interaction between magnetic fields and electric charges. According to the Lorentz force law, a magnetic field exerts a force on a charged particle that is in motion. The force (\( \mathbf{F} \)) experienced by a moving charge (\( q \)) in a magnetic field (\( \mathbf{B} \)) is given by the equation: \[ \mathbf{F} = q(\mathbf{v} \times \mathbf{B}) \] where \( \mathbf{v} \) is the velocity of the charge and \( \times \) denotes the cross product. This means that the force is dependent on the velocity of the charge and the direction of the magnetic field. 3. **Why Option B is Correct**: - Since a magnetic field exerts a force only on moving charges, the presence of a force on a moving charge indicates that a magnetic field exists at that point. If a charge is stationary, it does not experience any magnetic force, as the magnetic force depends on the velocity of the charge. ### Analysis of Other Options: - **Option A: Exerted on a stationary charge at the point**: - This option is incorrect because a stationary charge does not experience a magnetic force. The magnetic field interacts with charges only when they are in motion. Therefore, this option does not satisfy the condition for the existence of a magnetic field. - **Option C: Deflected at the point**: - While it is true that a moving charge can be deflected by a magnetic field, the term "deflected" is vague and does not directly indicate the presence of a magnetic field. Deflection can occur due to other forces as well, so this option is not specific enough to define the existence of a magnetic field. - **Option D: Strengthened at the point**: - This option is misleading. The term "strengthened" does not accurately describe the interaction between a magnetic field and a charge. A magnetic field can have varying strengths, but the existence of a magnetic field is not defined by whether it is "strengthened" or not. This option does not relate to the fundamental definition of a magnetic field. ### Summary of Key Points: - A magnetic field exists at a point if it exerts a force on a moving charge. - The force on a moving charge in a magnetic field is described by the Lorentz force law. - Stationary charges do not experience magnetic forces, making Option A incorrect. - The terms "deflected" and "strengthened" do not adequately define the existence of a magnetic field, making Options C and D weaker. ### Revision Summary: - A magnetic field exerts a force on moving charges, confirming its existence. - The Lorentz force law describes the interaction between magnetic fields and moving charges. - Stationary charges do not experience magnetic forces, ruling out Option A. - Clarity in terminology is crucial; vague terms like "deflected" or "strengthened" do not define magnetic fields effectively.