Explanation: In a resonant cavity, electromagnetic fields interact with the conducting walls, obeying specific boundary conditions. For an ideal conductor, the electric field (E-field) lines must always be perpendicular to the surface of the conductor. This is because any tangential component of the E-field would induce current and be short-circuited by the conductor itself. Therefore: * **B) E-lines are perpendicular to the end walls.** This is correct. The "end walls" are the flat conducting surfaces at the top and bottom of the cavity. The E-field lines must terminate perpendicularly to these surfaces. * **A) E-lines are parallel to the top and bottom walls.** This is incorrect. A parallel E-field component would be shorted out by the conductive wall. * **C) H-lines are perpendicular to the side walls.** This is incorrect. Magnetic field (H-field) lines must always be parallel (tangential) to the surface of a perfect conductor, not perpendicular. * **D) H-lines are circular to the end walls.** While H-field lines often form closed loops that can be circular within the cavity, the fundamental boundary condition at a conductor surface is that H-field lines are *parallel* to the surface. "Circular to" is less precise and doesn't directly describe this essential boundary condition.