How is a debenture typically authenticated by a company?

The correct answer is option 3- Under the common seal of the company.

A debenture is typically authenticated by a company under its common seal. This means that the company's official seal or stamp, which bears its name and registration number, is affixed to the debenture document. This process is a formal and legally recognized way of confirming the authenticity and validity of the debenture, signifying the company's commitment to the terms and conditions outlined in the debenture instrument.