Final answer:
The key performance indicator for 40% of users completing all sign-up tasks on a fitness app is the 'User Onboarding Completion Rate' or 'Completion Rate'.
Step-by-step explanation:
The key performance indicator (KPI) that demonstrates the measurement of 40% of users completing all of the sign-up tasks on a fitness app prototype would likely be the 'User Onboarding Completion Rate' or simply 'Completion Rate'. This KPI reflects the effectiveness of the app's sign-up process and indicates the proportion of users who have successfully navigated through the initial setup tasks.