Explanation

'Fused into an integrated whole' means blended or melted together into a single entity, which is precisely what must happen to divergent historical perspectives. 'Be made' is too vague to convey integration, and 'fit' is weak and normally intransitive in this sense ('fit into'). 'Be balanced' would preserve the views as separate but equal, the opposite of integration. Hence (b) 'be fused' is correct.