Final answer:
The best custom name for the sum of the amount field indicated as 'sales (000s)' is 'SalesTotal', which clearly signifies the total sales figures.
Step-by-step explanation:
When asked to use sales (000s) as the custom name for the sum of amount field, you should look for a name that indicates that it's a total of sales figures, usually in the thousands due to the '(000s)' notation. In this context, options A and B are less specific, whereas options C and D include the idea of a total amount. However, the best fit according to the question is option D: SalesTotal. This name effectively communicates that the field represents the total sum of sales figures.