To find the discrepancy between lines 5 and 6 of the reconciliation worksheet, we need to determine the difference between the two amounts:
- Line 5 total: $308.38
- Line 6 total: $328.37
Now, let's calculate the difference:
\[ \text{Difference} = \text{Line 6} - \text{Line 5} \] \[ \text{Difference} = 328.37 - 308.38 = 19.99 \]
Thus, the amount of the single transaction that Yolanda should look for to resolve the problem with the reconciliation is $19.99.
Therefore, the correct answer is: b. $19.99.