Darwin's Hypothesis of Evolution by Natural Selection included the observation that populations often grow faster than the food supply and therefore individuals need to compete for resources. Darwin borrowed this idea from the work of:

Thomas Malthus on  his essay on population.

Explanation:

Malthus concluded that human population grow faster than the available resources in their environment.However,no matter how this population increases,selection pressure like famine,natural disasters,always  keep the population in check,thus  maintaining the population size within moderate levels.

He believed the weakest in the population (with little resistance to the selection pressure) will die,thereby reducing the population, encouraging the survival of the fittest,and stabilizing and preventing expulsion.

Darwin, assumed that the surviving  organisms in the population must have inherited certain characteristics/traits  from their parents,which make them to resist the selection pressures.He called the surviving ones the survival of the fittest by natural selection.

i think the answer is b.

d. planting trees

the trees take in carbon to make oxygen.