Question

which method of gathering data will be most effective in learning how diet and fitness choices affect long - term health?
an experiment
an observational study
a sample survey
a census

Explanation:

An experiment allows for control over variables. In studying diet and fitness on long - term health, we can manipulate diet and fitness factors in an experimental setting while controlling other variables to isolate their effects. An observational study just observes without intervention, a sample survey asks for self - reported data which may be inaccurate, and a census is a complete count of a population and not ideal for studying cause - effect relationships regarding diet and fitness on health.

Answer:

an experiment