Explanation: Good insulators at radio frequencies (RF) are materials that minimize energy loss when exposed to alternating electric fields. This is crucial for components like capacitors, standoffs, and coaxial cable insulation to prevent signal degradation and heating. Materials like **Pyrex** (a type of borosilicate glass) and **mica** (a naturally occurring mineral) are excellent RF insulators. They exhibit low dielectric loss, meaning they absorb very little RF energy. Mica is particularly valued for high-frequency capacitors due to its stability and low loss tangent. Similarly, **Isolantite** (a ceramic material), **steatite** (another ceramic based on magnesium silicate), and **polyethylene** (a common plastic) are all well-known for their superior insulating properties at radio frequencies. They have low dielectric constants and very low loss tangents, making them ideal for high-frequency applications where minimal energy absorption is critical. **Rubber** can be an insulator, but many types exhibit higher dielectric loss at RF compared to the materials listed in A and B, leading to signal attenuation and heating. **Porcelain**, while a decent insulator, may not always offer the same low-loss characteristics as specialized RF ceramics like Isolantite or steatite, especially at higher frequencies or under specific conditions. Therefore, options A and B both list excellent choices, making D the comprehensive correct answer.