The correct answer is: C. Mica
Mica capacitors are mainly used for radio frequency tuning. They are made of a thin sheet of mica, which is a dielectric material with very high dielectric constant. This means that they can store a lot of charge for their size, which makes them ideal for use in high-frequency circuits.
Paper capacitors are made of two sheets of paper separated by a thin layer of aluminum foil. They are less expensive than mica capacitors, but they have a lower dielectric constant and are not as good for high-frequency applications.
Air capacitors are made of two metal plates separated by a small gap of air. They have the lowest dielectric constant of any type of capacitor, but they are also the most stable and reliable. They are often used in power supplies and other applications where stability is important.
Electrolytic capacitors are made of a thin layer of aluminum foil coated with an oxide layer. The oxide layer acts as the dielectric material, and the aluminum foil acts as the plates. Electrolytic capacitors have a high capacitance for their size, but they are also very sensitive to voltage and can be damaged if they are over-voltaged.
In conclusion, mica capacitors are the best choice for radio frequency tuning because they have a high dielectric constant and are stable at high frequencies.