b. scientist traps fish and
estimates the population of male and female fish based on the ratios of fish he trapped.
The key word is estimate. Estimate means that he gives a rough number. There aren't exactly that number of fish.
A. and C. are both exact numbers because the scientists manually count each one. This means these are direct observations, and therefore not the answer.
Hope this helped!
~Just a girl in love with Shawn Mendes