The correct answer would be: C. Ladrona
Héctor es ladrón y Cecilia es ladrona.
Héctor is a thief and Cecilia is a thief.
Option A would be wrong since ladra means to bark. If used in this sentence it wouldn't make much sense.
Option B would fit into this sentence since ladrón is thief but Cecilia is feminine not masculine.
Option D means to bark but in its stem form and wouldn't fit into this sentence.
Option C would be your best choice since it is feminine and fits into the sentence.
Hope this helps! :3