Also, another cause of the lack of political focus during Reconstruction was the great economic prosperity in the North following the Civil War. ... The failure of the North to effectively rebuild the South and bring it back into the Union during Reconstruction is evident after the time period.
The correct answer is one main clause.
This means that there is only one verb in the sentence, which in this case is the verb 'was.' Even though drive is also a verb, in this context it is used as an adjective driven (participle form of the verb to drive) and is thus not considered to be a verb. This means that there are no subordinate clauses but rather just one main clause.