To answer this question, we can use the federal tax table provided.
First, we need to find the row in the table that corresponds to Mario's gross biweekly earnings of $784.21. In this case, the row that matches his earnings range is the second row, which is labeled "If the wages are at least $760 but less than $780".
Next, we need to find the column that corresponds to the scenario of Mario claiming 1 more withholding allowance. Since Mario currently claims some number of withholding allowances, we can assume that the amount of income tax withheld would be found in the column labeled "And the number of withholding allowances is 1, the amount of income tax withheld is".
Looking at this column and finding the entry in the row that matches Mario's earnings, we see that the amount of income tax withheld is $68.
Now, let's calculate the difference in income tax withheld by claiming 1 more withholding allowance. This is given as $13 in the question.
By subtracting $68 (the amount of income tax withheld with 1 withholding allowance) from $68 + $13 = $81 (the amount of income tax withheld with 2 withholding allowances), we can determine how much more income tax would be withheld with 2 allowances.
$81 - $68 = $13
So, by claiming 1 more withholding allowance, Mario would have $13 more in his take-home pay.
Since we have found that claiming 1 more withholding allowance results in $13 more in take-home pay, we can conclude that Mario currently claims 3 withholding allowances, as stated in option a.