The theme of the World No Tobacco Day 2015, as celebrated by the World Health Organization, was "Stop illicit trade of tobacco products." This means that the focus of this year`s campaign was on raising awareness about the illegal trade of tobacco and the need for stronger measures to control it.

Option 1, "Raise taxes on tobacco," is not the correct answer. While increasing taxes on tobacco is a common strategy to reduce tobacco consumption, it was not the specific theme of World No Tobacco Day 2015.

Option 3, "Ban tobacco advertising, promotion, and sponsorship," is also not the correct answer. While restricting tobacco advertising is an important measure to reduce tobacco use, it was not the theme of World No Tobacco Day 2015.

Option 4, "Tobacco-free youth," is not the correct answer either. Although preventing tobacco use among youth is a crucial objective in tobacco control, it was not the specific theme of World No Tobacco Day 2015.

Therefore, the correct answer is option 2, "Stop illicit trade of tobacco products," as it accurately represents the theme of the World No Tobacco Day celebrated in 2015.