First, we need to convert the maximum weight that the mail carrier is allowed to carry from pounds to ounces:
30 pounds = 30 x 16 ounces = 480v ounces
Then, we can divide the maximum weight by the weight of one letter:
480 ounces / 12 ounces per letter = 40 letters
Therefore, the mail carrier is able to carry 40 letters.
So, the correct answer is option C: 40 letters.