Francis grew some plants for a science experiment. the sketch of his experiment after several days is shown

since there are no options posted i'll give you a few examples.

a dolphin's flipper, bird's wing, cat's leg.

the answer is true when observing or determining population size, a direct observation is use, in which each organism is counted in a population or in an area. one benefit of this measure is that you get the exact population count but it will take a long time to  determine  the  size  of a  population.