A paper capacitor is usually available in the form of

The correct answer is: A. tubes.

A paper capacitor is a type of capacitor that uses paper as the dielectric. The paper is impregnated with a dielectric fluid, such as mineral oil, to improve its insulating properties. Paper capacitors are typically available in the form of tubes, with the paper dielectric sandwiched between two metal foil plates. The tubes are then sealed and filled with the dielectric fluid.

Paper capacitors are used in a variety of applications, including power supplies, filters, and oscillators. They are characterized by their high capacitance, low leakage current, and good stability.

The other options are incorrect because:

• Option B, rolled foil, is not a common form factor for paper capacitors.
• Option C, disc, is a common form factor for ceramic capacitors, but not for paper capacitors.
• Option D, meshed plates, is a common form factor for electrolytic capacitors, but not for paper capacitors.