Demography is the scientific study of human populations, focusing on their size, structure, and distribution. It encompasses various aspects related to population dynamics, including birth rates, death rates, migration patterns, and population density. In the given options, the elements that fall under demography are:

P. Study of population density
S. Fertility rate

Let's explore each of these elements in detail:

1. Study of population density:
Population density refers to the number of individuals living in a specific area or region. It is calculated by dividing the total population of an area by its land area. The study of population density helps demographers understand the spatial distribution of populations, the availability of resources in different areas, and the impact of population growth on the environment.

2. Fertility rate:
Fertility rate is a measure of the number of live births per 1,000 women of childbearing age (usually between 15 and 49 years) in a given population during a specific time period. It provides insights into the reproductive behavior of a population and helps demographers analyze trends in population growth. Fertility rates are influenced by various factors, including cultural norms, socioeconomic conditions, access to healthcare, and government policies.

On the other hand, the elements that do not fall under demography are:

Q. Mortality rate:
Mortality rate refers to the number of deaths in a population during a specific time period, usually expressed as a rate per 1,000 individuals. While mortality rates are important indicators of public health and are closely related to population dynamics, they do not fall under the domain of demography.

R. Study of landforms in a country:
The study of landforms in a country falls under the field of geography, not demography. Geography focuses on the physical characteristics of the Earth's surface, including landforms, climate, vegetation, and natural resources.

In conclusion, demography primarily focuses on the study of population density and fertility rates, as they are key factors in understanding population dynamics. Mortality rates and the study of landforms are not directly related to the field of demography. Therefore, the correct answer is option C: Only P, Q, and S.