A semicolon is the best choice for this sentence. The first half of the sentence "Jonathan was not particularly hungry" is a complete idea. The second half of the sentence "he spent the meal picking at the food on his plate" is also a complete idea. When joining two complete ideas, a comma is not enough punctuation. It needs to have a conjunction as well. A semicolon is enough punctuation by itself and should not be paired with a coordinating conjunction.
The answer to your question would be option B. This would construct the sentence into, "Jonathan was not particularly hungry; he spent the meal picking at the food on his plate".
