Answer:
B. 0.06 ounce
Explanation:
The mixture prepared by Chuck is enough for 14 plants.
If Chuck only used it, according to the recommended amount, for 12 plants, then only 12/14 parts of the 0.42 ounces of plant food were used and 2/14 parts still remain the watering can. The amount of plant food left, in ounces, is:

0.06 ounce is left in the watering can.