The correct answer is option 1 - cell division. Cell division is the process through which a unicellular organism reproduces. Unicellular organisms, as the name suggests, consist of a single cell. In order to reproduce, they undergo cell division, where the parent cell divides into two or more daughter cells. This process allows the organism to create copies of itself and increase its population.

During cell division, the parent cell duplicates its genetic material and distributes it equally into the daughter cells. This ensures that the genetic information is passed on to the offspring. Following the division, each daughter cell has the same genetic composition as the parent cell.

It is important to note that cell division in unicellular organisms is a form of asexual reproduction. Asexual reproduction does not involve the fusion of gametes or the involvement of another organism. Instead, the offspring are genetically identical to the parent cell. This type of reproduction is advantageous for unicellular organisms as it allows for rapid and efficient multiplication of their population.

In summary, unicellular organisms reproduce through cell division, where the parent cell divides into genetically identical daughter cells. This process enables them to increase their population and maintain their existence.