The concept of `survival of the fittest` refers to the idea that individuals who are best adapted to their environment are more likely to survive and reproduce. Option 1, Oparin, is incorrect as he is known for his work on the origin of life, not the concept of `survival of the fittest`. Option 2, Darwin, is the correct answer. Charles Darwin proposed the theory of evolution through natural selection, which is the foundation for the concept of `survival of the fittest`. Option 3, Spencer, is also incorrect as Herbert Spencer coined the phrase `survival of the fittest`, but he did not advocate it as a scientific concept. Option 4, Haeckel, is incorrect as Ernst Haeckel contributed to the fields of embryology and evolutionary biology, but he did not advocate the concept of `survival of the fittest`.