A study was conducted to examine how a healthy lifestyle is related to a life expectancy that is free from major chronic diseases. Participants were a group of women born in 1950. The exposure factors were never smoking, body mass index, physical activity, and moderate alcohol intake. The study started in 1970 and the participants were followed-up over the years to measure the outcomes both from those who were exposed and those who were not. What is the most likely design of this study?

a. Cross-sectional

b. Prospective cohort

c. Randomised controlled trial

d. Case-control