Hint: Struggle for existence and the survival of the fittest is that law of evolution which is mentioned in Darwin's book. The concept of the struggle for existence concerns the competition or battle for resources needed to live and natural selection is the process by which organisms change over time as a result of changes in heritable physical or behavioral traits.
The struggle for existence: The concept of the struggle for existence concerns the competition for resources needed to live. Darwin reasoned that there must be a competition between individuals belonging to the same or other species, for sharing food, water and living space. Every individual puts efforts for fulfilling the basic needs such as space for living, food for a living, mate for reproduction and also protection from enemies. This competition is known as 'struggle for existence'.
Survival of the fittest: Only those organisms, which have favorable variations, are at a specific advantage over others or the organisms best adjusted to their environment are the most successful in surviving and reproducing. Such individuals survive the struggle for existence while those, which are less adapted or not adapted, get eliminated.
So, the first option is Lamarckism, which is a notion that an organism can pass on to its offspring physical characteristics that the parent organism acquired through use or disuse during its lifetime. So, this option is wrong. Our next option is the Oparin hypothesis which suggests that life arose gradually from inorganic molecules, with “building blocks” like amino acids forming first and then combining to make complex polymers. Our 3rd option is Mendelism, which is the theory of hereditary which suggested that individual characteristics were determined by inherited `factors', so this option is also wrong. Option D is Darwinism. Darwinism is a theory of biological evolution stating that all species of organisms arise and develop through the natural selection of small, inherited variations that increase the individual's ability to struggle, survive, and reproduce. So this option is correct.
The correct option is, D) Darwinism
Note: Darwin wrote the book ' the origin of the species'. In which he wrote about "Struggle for existence" and "Survival of fittest". On the Origin of Species, published on 24 November 1859, is a work of scientific literature by Charles Darwin which is considered to be the foundation of evolutionary biology. Darwin's book introduced the scientific theory that populations evolve over the course of generations through a process of natural selection.