Key Concept: Population-counting methods differ based on whether the organism is mobile or stationary.
Why it's correct: Mark and Recapture is the standard method for mobile animals like fish — catch, tag, release, then recapture to estimate total stock size from the recapture ratio.
Why others fail: Transect and Quadrat sampling suit stationary organisms like plants or benthic fauna, and radiography is an imaging tool, not a population-counting method.