'Ebb and flow' is a compound subject with the coordinating conjunction 'and.' Compound subjects joined by 'and' are usually treated as plural and need plural verbs. However, in this sentence, 'ebb and flow' is considered as a single unit or entity, hence it is treated as a singular subject and requires a singular verb. So, the correct option is A) was.
Options C and D do not result in grammatically correct sentences.