We expect the height of a helium balloon to be increasing, up to the point where it bursts.
The only graph that shows consistently increasing altitude with a final drop is graph D.
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<em>Additional comment</em>
The graph of D shows the altitude increases at an increasing rate. The buoyancy force decreases with altitude, so we expect the balloon would actually have a decreasing rate of altitude increase. The actual curve would likely be concave downward.
