Concept:
Assertion--Reason questions require determining whether each statement is true and whether the reason correctly explains the assertion.
Step 1: Examine Assertion (A).
Rapid population growth increases pressure on available resources, employment opportunities, housing, education and healthcare.
Therefore,
\[
\boxed{\text{Rapid population growth is one of the causes of poverty in developing countries.}}
\]
Hence,
\[
\boxed{\text{Assertion (A) is Correct.}}
\]
Step 2: Examine Reason (R).
Many developing countries mainly export primary commodities such as agricultural products, minerals and raw materials.
Therefore,
\[
\boxed{\text{Reason (R) is Correct.}}
\]
Step 3: Determine whether (R) explains (A).
Although both statements are individually true, the dominance of primary commodity exports does not explain why rapid population growth causes poverty.
The two statements relate to different aspects of development economics.
Hence,
\[
\boxed{\text{Reason (R) is not the correct explanation of Assertion (A).}}
\]
Therefore,
\[
\boxed{\text{Correct Answer}=(B)}
\]