The expression given for the salesperson's total earnings is \( 2000 + 0.1s \).
In this expression:
- \( 2000 \) is a constant amount added to the earnings.
- \( 0.1s \) represents the commission, which is 10% (or 0.1) of the sales amount \( s \).
Since the base salary is typically a fixed amount that the salesperson earns regardless of sales, and given that this fixed amount is added to the commission, we can conclude that:
A. his monthly base salary in dollars.
So, the correct answer is A.