What type of film base was primarily used during World War I?

During World War I, the primary type of film base used was cellulose nitrate. This film was known for its high sensitivity and ability to produce sharp images, which made it popular for motion pictures during that era. Although glass plates were indeed used in the earlier days of photography, they began to fall out of favor as flexible film technology advanced. Cellulose nitrate film allowed for greater portability and ease of use on the battlefield and in various photographic applications.

Safety film, on the other hand, became more prevalent after World War I due to concerns regarding the flammability of nitrate film. Polyester film came into use later in the mid-20th century and is known for its durability and longevity, but it was not available during World War I. Therefore, cellulose nitrate is the correct choice as it was the dominant film base used during that period.