The most suitable option for Cumulus Cloud Bank to provide its customer service staff with real-time access to customer accounts, including the ability to view current transactions and balances, is to embed custom Lightning web components on the Account and Financial Account pages, and use custom Apex to make callouts to the core system services. This option allows the bank to leverage the Lightning Platform features and capabilities, such as Lightning web components, Apex, and callouts, to create a seamless and performant integration with their core systems. By using custom Lightning web components, the bank can create custom user interfaces that display the data from the core systems on the Account and Financial Account pages. By using custom Apex, the bank can write code that makes callouts to the core system services using HTTP requests and responses, and handle any authentication, error handling, or data transformation logic.