Indeed it´s an interesting question.
With multiple choice questions, a point is that the answer that best matches what it´s asked is a candidate to be the one.
Another point with a multiple choice questions is that the answer (not only the correct one) but the one that stands true all of the time, in all cases, and without exception. So, it is the stronger or that covers the matter most.
"continuous" would throw me on the rocks, I must admit. Because it may mean "pattern". But even so, if Wordreference example is something to go by, it is more apt to be used with things, not relating to time like in the question. But, it´s me here, I could be mistaken, of course.
Plus, here I have the benefit of hindsight (read - search, to know which answer is, etc).http://www.wordreference.com/enpt/continuous
Something that happens periodically, at intervals (with time between, not continuously/with spaces between.)
Periodic adj (recurring regularly) periódico adjhttp://www.wordreference.com/enpt/periodic
I looked up "ongoing" at Wordreference to see what gives..., but then not much on that one (on my opinion).
Then Thesaurus came to my rescue:https://www.thesaurus.com/browse/ongoing
Hmm, then continuing, in process, growing... Yes! It makes sense. This seems like passing the test of "is true all the time".
But I am not going to say that I would score that one. As I said, now I have the benefit of having time to think, and to think it through.
That´s a moment (to me) that the one that is a reader wins, but then he has to be lucky as well. To the word he knows well to be on the exam!