Which of the Following Scenarios Would Provide the Most Legitimate

Which of the following scenarios would provide the most legitimate data on population density?

A) Count the number of nests of a particular species of songbird and multiply this by a factor that extrapolates these data to actual animals.
B) Count the number of pine trees in several randomly selected 10 m x 10 m plots and extrapolate this number to the fraction of the study area these plots represent.
C) Use the mark-and-recapture method to estimate the size of the population.
D) Calculate the difference between all of the immigrants and emigrants to see if the population is growing or shrinking.
E) Add the number of births and subtract the individuals that die to see if the population's density is increasing or decreasing.