Of course, we're not going to use patiently because the ~ly indicates an adverb (adverbs are modifiers to verbs, and there's clearly no action being preformed here which is being done in a patient manner or anything like that)
Here, we are comparing Seth to Ben. Because of this, we use the comparative form, more patient. An important part of that is how it can be followed by than.
The most patient form, which is superlative, says that Seth would be more patient than anyone else in an aforementioned group, so this doesn't apply to this sentence. You could use it in something like "Seth is the most patient out of all of his friends."
C. more patient
