One study found that the best salespersons have four traits in common: intrinsic motivation, a disciplined work style, the ability to close a sale, and the ability to build relationships with customers. knowing this would be most useful at which stage of sales force management?

Recruiting and selecting of salespeople.

Explanation:

Based on the information provided within the question it can be said that this information would be most useful during the Recruiting and selecting of salespeople. This is because at this stage knowing this information would allow a company to choose the absolute best salespeople, thus dramatically increasing the revenue of the company, since these individuals will close far more deals than someone without these traits.

Looking at it from the information in the Question, I would say it is most useful during the "Selection Stage".

Explanation:

I say its important during the selection stage, where the company select and subsequently hire suitable people for the job.

But remember this, Identifying intrinsic motivation, Work style, identifying ability to close sales and how they build customer relationships is extremely difficult.

Some of these traits are only visible when the selected employees begin to work! Although if the employees have had experience in the past, you can easily identify them. Just gotta look at their track record.