Answer:
C. Although.
Step-by-step explanation:
Conjunctions are words that we use to join two or more sentences together. They are used in correspondence to the sentence, depending on how the sentence is structured, and what it is implying.
In the given two sentences "he was no longer hungry" and "James ate a large piece of cake", we can join these two by using the conjunction "although". "Although" is used to join two contrasting sentences to make a single sentence. And depending on the structure of the sentences, it can be placed in the beginning or middle of the sentence.
Thus, the final sentence will be
Although he was no longer hungry, James ate a large piece of cake.