Which antenna is a wideband antenna?

Option A: Hertz antenna
The Hertz antenna, also known as a dipole antenna, operates efficiently over a narrow frequency range. It is not considered a wideband antenna since its bandwidth is limited.

Option B: Marconi antenna
The Marconi antenna is a type of monopole antenna typically used for narrowband applications. It is not designed to handle a wide range of frequencies, making it unsuitable as a wideband antenna.

Option C: Folded dipole
The folded dipole is a type of wideband antenna. Its construction allows it to operate over a broader frequency range compared to standard dipole antennas. This characteristic makes it suitable for applications requiring wideband operation.

Option D: None of the above
This option is incorrect because the folded dipole antenna is explicitly a wideband antenna. Therefore, this statement does not apply.

Conclusion:
The correct answer is Option C: Folded dipole, as it is specifically designed to function as a wideband antenna.