Final answer:
Phosphorus is the additive filled into cellulosic materials to increase their fire resistance by reducing their tendency to smolder.
Step-by-step explanation:
The additive filled into cellulosic materials to increase their fire resistance by reducing their tendency to smolder is Phosphorus (A).
Phosphorus is known to burn rapidly in air, but it will burn even more rapidly if the concentration of oxygen is higher.
Adding phosphorus to cellulosic materials increases their fire resistance by reducing the availability of oxygen, which hinders the combustion process.