To calculate the percentage of apples that were bad, Vincenzo needs to divide the number of bad apples (4) by the total number of apples (34) and then multiply by 100 to convert it to a percentage.
The correct method is:
C: Divide 4 by 34, then multiply by 100 and add a percent sign.
So, the calculation would be:
\[ \left(\frac{4}{34}\right) \times 100 \approx 11.76% \]
Therefore, the answer is C.