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The haploid state is defined as containing one complete set of chromosomes, which is characteristic of gametes (sperm and egg cells) in organisms that reproduce sexually. In this state, the cells have half the number of chromosomes compared to diploid cells, which contain two sets of chromosomes (one from each parent).

This is crucial for sexual reproduction because when two haploid gametes unite during fertilization, they form a diploid zygote, restoring the full set of chromosomes for the organism's next generation. This setup is essential for maintaining the correct number of chromosomes through successive generations.

The other options describe states that do not pertain to haploids. For instance, somatic cells are diploid and contain pairs of chromosomes, which is contrary to the haploid state. Genetic variation can be influenced by haploidy, particularly through processes like recombination during meiosis, but it does not describe the state itself. Therefore, the statement that best captures the essence of the haploid state is that it contains one complete set of chromosomes.