Final answer:
Fire can spread vertically in commercial and residential buildings known as Taxpayers through concealed spaces, ducts, and shafts, but open stairs and trap doors typically are not a path for fire spread as they are designed for egress and made with fire-resistant materials.
Step-by-step explanation:
Fire may spread vertically in Taxpayers, which are typically commercial buildings with residential units above, through several pathways, but not all pathways facilitate fire spread equally. When considering the options given, option B, 'open stairs or trap-doors' is incorrect. This is because open stairs and trap doors do not conceal space and provide a direct pathway for fire to travel, unlike concealed spaces where fire can go unnoticed and spread undetected.
Fire often spreads through concealed spaces like those between furred plaster and brick walls, mentioned in option A, as well as via ducts such as air conditioning, heating, and ventilation ducts (option C), and light and ventilation shafts (option D). These areas are prone to accumulating combustible materials and can channel heat, smoke, and flames between floors. However, open stairs and trap doors are typically designed as part of the egress path and should be equipped with fire-resistant materials and design elements that inhibit the spread of fire, making them less likely to facilitate the vertical spread of fire compared to the other listed pathways.