You are correct, A is the appropriate choice in this case.
Notice in your example, "Most fish have fins" that most precedes a plural noun, fish. In this case, the use of most is correct. However in the original statement(The most important development of the Neolithic age), most would precede an adjective describing a singular noun.
Most by itself is usually used to describe a group or large portion. In order to use most to describe the ultimate or primary reason, it must be preceded by an indefinite article (like "the").