Introduction:
Charles Darwin, the renowned naturalist, developed the theory of natural selection to explain the process of evolution. During the formulation of this theory, he was influenced by several factors. However, the most significant influence on Darwin's theory of natural selection was the essay written by Thomas Malthus on population.

Explanation:
- Essay of Malthus on population: Thomas Malthus, an economist, wrote an essay in 1798 titled "An Essay on the Principle of Population." In this essay, Malthus argued that population growth would always surpass the available resources, leading to competition for survival. He suggested that individuals with favorable traits would have an advantage in this competition and would be more likely to survive and reproduce. Malthus's ideas of population growth and competition greatly influenced Darwin's thinking. Darwin realized that in nature, there is a struggle for existence and only the fittest individuals survive and pass on their traits to the next generation. This concept formed the basis of Darwin's theory of natural selection.

- Cell theory: The cell theory, formulated by Matthias Schleiden and Theodor Schwann, states that all living organisms are composed of cells, and cells are the basic units of life. While the cell theory was a significant scientific advancement, it did not directly influence Darwin's theory of natural selection. Darwin's theory focused on the mechanisms of evolution and the process of natural selection, rather than the cellular structure and function.

- Mendel's laws of inheritance: Gregor Mendel, an Austrian monk, conducted pioneering experiments on pea plants and formulated the laws of inheritance. His work laid the foundation for the field of genetics. However, Mendel's laws of inheritance were not known to Darwin during the formulation of his theory of natural selection. Mendel's work remained largely unknown until the early 20th century when it was rediscovered by scientists.

- None: The option "None" is incorrect as Darwin was indeed influenced by the essay of Malthus on population. This influence shaped Darwin's understanding of the struggle for existence and the role of competition in the process of natural selection.

Conclusion:
In conclusion, the essay of Thomas Malthus on population was the primary influence on Charles Darwin's theory of natural selection. Malthus's ideas of population growth, competition, and the struggle for existence provided Darwin with a framework to explain the process of evolution through natural selection. While other scientific advancements such as the cell theory and Mendel's laws of inheritance were significant, they did not directly impact Darwin's formulation of the theory of natural selection.