Final answer:
In a cohort study, the characteristic used to form the cohort is the age of the individuals. In this case, the scientist collected health data on a group of people born in 1976, forming a cohort based on their shared birth year.
Step-by-step explanation:
In a cohort study, the characteristic used to form the cohort is Age. The scientist collected health data on a group of people born in 1976, meaning that this group shares the same birth year and thus the same age cohort. This allows the scientist to study how health outcomes may differ among individuals of the same age.