Which of the following contributes to increase in population density ?
(a) Natality
(b) Mortality
(c) Immigration
(d) Emigration

Options

A. a, b
B. b, c
C. c, d
D. a, c

Answer

To determine which factors contribute to an increase in population density, we need to analyze each option provided:

1. **Understanding Population Density**:
- Population density refers to the number of individuals of a species living in a given area. An increase in population density means that there are more individuals per unit area.

2. **Analyzing Each Option**:
- **(a) Natality**: This term refers to the birth rate of a population. An increase in natality means more individuals are being born, which directly increases the population size and, consequently, the population density. Thus, natality contributes to an increase in population density.

- **(b) Mortality**: Mortality refers to the death rate within a population. An increase in mortality means more individuals are dying, which decreases the population size and, therefore, decreases the population density. Thus, mortality does not contribute to an increase in population density.

- **(c) Immigration**: Immigration is the process where individuals move into a population from another area. This influx of individuals increases the total number of individuals in the population, thereby increasing the population density. Thus, immigration contributes to an increase in population density.

- **(d) Emigration**: Emigration is the process where individuals leave a population to move to another area. This results in a decrease in the number of individuals in the population, leading to a decrease in population density. Thus, emigration does not contribute to an increase in population density.

3. **Conclusion**:
- The factors that contribute to an increase in population density are **(a) Natality** and **(c) Immigration**. Therefore, the correct answer includes both options A and C.

### Final Answer:
The factors that contribute to an increase in population density are (a) Natality and (c) Immigration.