Final answer:
In most species, genetic sex determination is responsible for sex ratios of approximately 1:1, emerging from the chromosomal mechanisms that determine the sex of the offspring. Option a.
Step-by-step explanation:
In most species, the process responsible for sex ratios of approximately 1:1 is genetic sex determination. This mechanism typically involves chromosomes that determine the sex of the organism. For example, in mammals, sex is determined by the presence of XX chromosomes (females) and XY chromosomes (males). This system tends to produce approximately equal numbers of male and female offspring, ensuring a balanced sex ratio in the population.
On the other hand, frequency-dependent selection can affect sex ratios when individuals with either commonplace or rare traits are favored. Sex ratios can be skewed in some species due to this type of selection. Moreover, sexual selection can lead to what is known as sexual dimorphism, where males and females develop distinctly different appearances or behaviors as a result of different selective pressures related to reproduction. Though fascinating in its own right, sexual selection does not directly answer why sex ratios tend to be 1:1.