Which type of bomb was used to start fires in cities during World War II?

The use of incendiary air bombs during World War II was particularly notorious for starting fires in urban areas. These bombs were specifically designed to ignite fires upon impact, leading to widespread destruction in cities. One of the most significant examples of their use was during the firebombing campaigns in places like Tokyo and Dresden, where entire neighborhoods were engulfed in flames, resulting in immense civilian casualties and destruction.

While atomic bombs were used later in the war on Hiroshima and Nagasaki, their purpose was not to start fires in a conventional sense; rather, they aimed to cause massive devastation through nuclear explosion. Conventional bombs primarily aimed at explosive impact and fragmentation effects, not necessarily focused on setting fires. Thermobaric bombs, which use a different mechanism involving a fuel-air explosion, were not used in World War II but rather gained notoriety in more modern conflicts.

Thus, the incendiary air bomb stands out as the correct answer for this specific context of igniting fires in cities during the war.