Here HOWEVER implies contrast between two sentences. So if one sentence is positive, then the other should be negative or vice-versa.
Option A :
steadily : predictable, reliable, unchanging pace
stabilizes : calm, steady
No contrast.
Option B :
forever : continues
increases : getting big/large
No contrast.
Option C :
erratically : unpredictable
mitigates : lessen
Not either, Erratic is not suiting the sentence.
Option D :
indefinitely : continues/ for indefinite time
diminishes : lessen
Perfect.
Option D is the answer.