Concept:
According to urban settlement classifications, a megalopolis refers to a very large urban region formed by the growth and merging of metropolitan areas with extremely high population concentration and economic activity.
Cities such as Mumbai and Kolkata are often categorized under large metropolitan or megalopolitan urban regions.
Step 1: Analyze Assertion (A).
Mumbai and Kolkata are among the most densely populated and highly urbanized cities in India.
Due to their large urban spread and concentration of population and economic functions, they are considered megalopolitan regions.
Hence, Assertion (A) is correct.
Step 2: Analyze Reason (R).
The statement says:
\[
\text{Population range of Megalopolis is between 50 lakhs to 1 crore.}
\]
This is incorrect because megalopolis generally refers to urban regions with population far exceeding:
\[
1 \text{ crore}
\]
and involving interconnected metropolitan regions.
Therefore, Reason (R) is incorrect.
Step 3: Choose the appropriate option.
Since Assertion (A) is correct but Reason (R) is incorrect, the correct answer is:
\[
{(A)\; \text{is correct but}\; (R)\; \text{is not correct}}
\]