D is better, as it emphasizes the steadiness and continuity of the process; but C is possible.
Generally, "throughout" can be used only of periods (whereas "through" can be used inother contexts: 'he succeeded through sheer hard work', 'he spoke through an interpreter'. 'he blew into the trumpet through the mouthpiece'... etc.) And in expressions of periods, 'throughout' can be more accurately replaced by 'all through':
It was the same throughout his life.
It was the same all through his life.
It was the same through his life.
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