Concept:Rapid population growth occurs when birth rates remain very high while death rates decline.
Explanation:The major reason for fast population growth in Tropical Africa is the limited use of family planning.
Many families have more children because modern contraceptive methods are not widely available or accepted.
At the same time, better healthcare has reduced death rates, so more babies survive.
High birth rates combined with lower death rates cause the population to increase rapidly.
Improved transport facilities do not directly cause population growth.
Pressure on land is a result of population growth, not its main cause.
Malnutrition and diseases tend to slow population growth, not accelerate it.
Therefore, lack of family planning is the most important factor among the given options.
Answer:D. lack of family planning.