Final answer:
Della is nervous about her Christmas spending for Jim because she counts her money three times, which suggests anxiety about the amount being sufficient for his gift.
Step-by-step explanation:
The best evidence that Della is nervous about how much she has to spend on Jim for Christmas comes from the detail that Della counts the money three times. This action suggests a level of anxiety and a hope that the amount might somehow have changed or that she may have miscounted originally. The act of recounting indicates her concern about the sufficiency of the funds for a satisfactory Christmas present for Jim. The other options, such as having only a small amount of change, knowing the proximity of Christmas, or arguing with grocers, do not directly convey nervousness about the money available to spend on a gift.