Final answer:
The bone that fits the description of having a small flat plate perpendicular to the main plate is the vomer. It is a component of the nasal septum within the nasal cavity.
Step-by-step explanation:
The bone described as a small, thin, flat plate with an even smaller thin, flat plate perpendicular to the main plate most likely refers to the vomer. This bone is found within the nasal cavity and forms part of the nasal septum, which divides the nose into left and right nostrils. The vomer and its perpendicular plate would align with the description given. The other options, such as the scapula, sternum, and clavicle, while they are indeed flat bones, they do not possess a structure where a smaller flat plate is perpendicular to the main plate as described.