Answer: b. Your client has Business view selected in their settings
Explanation:
There are two views when it comes to Quickbooks Online: Business view and Accountant view. Accounting view is for more seasoned users who have some form of accounting training so it has more features.
Business view on the other hand is simplified for those who don't know much about accounting. It is easy to follow and some features are reordered for simplicity.
The Banking tab is one of those features and it is not openly shown in the business view so the most probably the reason your client can't see it is that she is using the Business view.