(a)(i) Explanatory Notes on Population Density: (i) Population density refers to the ratio of people per unit area. It can be expressed as Total population Total land area Population can be expressed as persons per km'. Population density could be high or low depending on the factors of increase or decrease.
(ii) Explanatory Notes on Birth Rate: Birth rate refers to the number of live births per thousand persons in a population. The birth rate is presented on a yearly or annual basis. Birth rate could be high or low depending on the factors of increase or decrease. It is used to estimate the annual growth rate of a population.
(iii) Explanatory notes on Optimum Population: Optimum population is the relationship between population and available resources. It is a situation where the number of people in a given area equals the resources within the area. Optimum population is a difficult situation to attain in real life. It is a situation of equilibrium. Any additional number of people leads to over population and any reduction in number of people leads to under population. This situation often involves size of population and technical competence.

(b) Ways by which the Problems of Under Population can be Solved: (i) Improved welfare packages. (ii) Discouraging all forms of birth control. (iii) Public enlightenment. (iv) Discouragement of monogamy. (v) Economic support for single parents.
(vi) Encouragement of early marriage. (vii) Child welfare benefits, such as monthly upkeep allowances. (viii) Encouragement of polygamy. (ix) Legislation against abortion. (x) Encouragement of immigrants. (xi) Acquisition of skills. (xii) Increase in food supply. (xiii) Government incentives, such as free medical care, education, etc. (xiv) Gainful employment.