Option 1, 1991: This option is incorrect. The H5N1 virus was not discovered in 1991.

Option 2, 1995: This option is incorrect. The H5N1 virus was not discovered in 1995.

Option 3, 1997: This is the correct answer. The H5N1 virus was first discovered in 1997. It was identified in birds in Hong Kong during an outbreak of avian influenza.

Option 4, 2001: This option is incorrect. The H5N1 virus was not discovered in 2001.

In summary, the H5N1 virus, which causes bird flu, was first discovered in 1997 during an outbreak in birds in Hong Kong.