There are two basic ideas that are key to darwin's theory of evolution. the first is that present day species are descendants of ancient ancestors, called "descent with modification" the second is

Natural selection is the driving force to explain the mechanisms  of  evolution.

Explanation:

◼In a given population Organisms usually produce more offspring than what the environment can support.  

◼ VARIATION, is driving force  for  the manifestation of  sustainable inheritable characteristics from parents in a given population.

◼ Organisms, who acquired certain characteristics from variation; tend to survive than others, and therefore pass these inheritable characteristics to their offspring.

◼There is competition for the available resources among the offspring which leads to a stable population in size after a period of time.

◼Darwin deducted that, variants of best adaptation will be selected for by natural conditions operating in the environment of the population at that particular period (natural selection).Therefore natural selection takes place, and the organisms with best variation will have selective advantage above others. Consequently they have higher survival rates compare to others in the population.

This is   survival of the fittest by natural selection , this is the second conclusion  of Darwin theory.

correct answer is a. and d.