You are correct!  The answer is "a.  Add bulky browns/carbon rich material."

Most odor problems often result from either too much moisture, which causes the compost to compact and loose oxygen flow or too much green/nitrogen rich material which off-gas smelly odors (or both). Adding bulky brown/carbon rich material will help absorb the excess moisture and nitrogen rich gasses. You can just add the browns to the top of the compost bin, but if odors persist, you may have to mix the browns in throughout the pile, where the bulky material will help keep the pile oxygenated.

For more information on why your pile may have odors, see "Troubleshooting - Odor".