Final answer:
The flammable film base used during WWI and coated on one side was nitrocellulose, also known as celluloid, which was later replaced by safer materials due to its flammability.
Step-by-step explanation:
During WWI, the film base that was highly flammable and only coated on one side was a) Nitrocellulose. This early form of film was made of celluloid, which is a compound created from nitrocellulose and camphor. Celluloids were commonly used in the movie and photography industries as film stock. However, due to its highly flammable nature, it was later replaced by acetate safety film.
Celluloid's other applications included manufacturing items like table tennis balls, musical instruments, and guitar picks, but its use has significantly declined over time.