Final answer:
The mass production of penicillin during WWII saved many soldiers' lives.
Step-by-step explanation:
The improvement made to penicillin during WWII that saved many soldiers' lives was its mass production. Before the war, penicillin was produced in small quantities and was not readily available. However, during the war, scientists developed methods to produce penicillin on a large scale, allowing it to be used as a widespread treatment for bacterial infections. This increase in production saved countless lives by providing an effective antibiotic to soldiers fighting infections on the battlefield.