Answer:
I think it's Naturalistic Observation
Step-by-step explanation:
Naturalistic observation is a research method that is used by psychologists and other social scientists. The technique involves observing subjects in their natural environment. It can be used if conducting lab research would be unrealistic, cost-prohibitive, or would unduly affect the subject's behavior.
So yeah if you want the best results for the "Length of marriage is related to greater life satisfaction” You should go with Naturalistic Observation, I'm not 100% sure about this but good luck on your test/assessment!