Answer: Flexible duct because, They also developed more than 1,000 safety criteria, some of which are... labelled goods bearing a silver UL sticker on the duct that defines this listing. ... lie in the research carried out by UL and, as a result, the authorized use of the code. Air Ducts are required to perform 15 separate tests.
Source: https://dundasjafine.com/ul-classification/
