Final answer:
During World War I, there was a significant change in the American workforce, with over a million joining the armed forces and a roughly three million increase in overall workforce due to the war effort, including around one million women entering new professions.
Step-by-step explanation:
The chart in question likely indicates a significant change in the composition of the American workforce during World War I. Specifically, the chart is interpreted to suggest that option (a) The number of people in the armed forces increased by more than one million is correct due to the massive national draft and the need for a larger military presence during wartime. Furthermore, historical data shows that union membership and overall employment increased during this period, aligning with option (b) The number of people in the workforce increased by about three million.
Women played a crucial role, with approximately 1 million American women entering jobs previously closed to them, showing the restructuring of the workforce and an increase in female employment. It is also noted that government efforts and the need for military and civilian production resulted in nearly full employment in the U.S., dropping unemployment significantly.