The correct option is B the most
The superlative degree of the adjective 'many' is "most". In this context, "the most" is the most suitable answer as the use of the definitive article "the" indicates that the subject is something in particular. Here, the context refers to a particular city in a particular country. So, it becomes "the most" famous city of that country. Hence Option B is the correct answer.
"The more" is incorrect as it is a comparative degree adjective, but here the comparison is not between a certain number of cities, but all the cities in America. Hence option A is incorrect.
"More" is a comparative degree adjective used while comparing between two quantities or objects. Hence option C is incorrect.
"Most" is a superlative degree adjective, but as discussed above, it is ineffectual without the determiner "the". Hence option D is incorrect.