Explanation: Pentode vacuum tubes include a screen grid positioned between the control grid and the anode (plate). This screen grid effectively shields the control grid from the plate, drastically reducing the grid-to-plate capacitance (Cgp). A low Cgp minimizes internal feedback, which is the primary cause of unwanted oscillations in RF amplifiers. Therefore, pentodes usually do not require neutralization (Option A) to remain stable, making them inherently more stable than triodes. However, a small residual Cgp still exists. At ultra-high frequencies (UHF), the inductive reactance of internal leads and components, combined with even tiny capacitances, can create resonant conditions. These resonances can lead to phase shifts that promote oscillation, despite the low Cgp. Consequently, careful design, shielding, and sometimes even neutralization may be required at UHF to ensure stability (Option C). Since both A and C accurately describe pentode behavior, D is the correct answer.