Import via Data management page (A):

Dynamics 365 Finance supports importing budget register entries using the Data management framework, which allows bulk import of budget data from external files or systems.

Microsoft Docs - Budget register entries

'Budget register entries can be imported using the Data management workspace for efficient bulk data entry.'

Manual entry on Budget register entry page (B):

You can directly create and edit budget register entries manually in the Budget register entry form within the system.

This is the basic and most straightforward method.

Microsoft Docs - Budget register entry form

'Manually create budget register entries by entering amounts and financial dimensions on the budget register entry page.'

Microsoft Office add-in (D):

Microsoft Office add-in integration allows budget planners to work in Excel, entering budget data and publishing it directly to Dynamics 365 Finance as budget register entries.

This is helpful for users comfortable with Excel for budget planning.

Microsoft Docs - Budgeting with Excel add-in

'Use the Microsoft Office add-in to create and publish budget register entries from Excel to Dynamics 365 Finance.'