**Core Concept**
The demographic bonus refers to the period in a population's life cycle when the dependency ratio (the ratio of non-working individuals, such as children and elderly, to working individuals) declines due to a decrease in fertility rates and an increase in life expectancy. This phenomenon is often seen in countries with low fertility rates and high life expectancy.

**Why the Correct Answer is Right**
The correct answer is related to the decline in fertility rates, which leads to a decrease in the number of children per working-age individual. This, in turn, results in a decrease in the dependency ratio. As the population ages, the number of elderly individuals increases, but the decrease in fertility rates leads to a lower number of children, ultimately causing the dependency ratio to decline. This is a key factor in the demographic bonus.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because an increase in life expectancy alone does not directly affect the dependency ratio, although it does contribute to the overall aging of the population.
**Option B:** This option is incorrect because an increase in mortality rates would actually lead to an increase in the dependency ratio, as more individuals would be dependent on working-age individuals due to premature death.
**Option C:** This option is incorrect because an increase in fertility rates would lead to an increase in the number of children, which would increase the dependency ratio, rather than decrease it.

**Clinical Pearl / High-Yield Fact**
The demographic bonus is often accompanied by a decline in the labor force and a potential shortage of skilled workers, which can have significant economic and social implications.

**Correct Answer: D. Decline in fertility rates.**