This doesn't help, because we have no reason to think the planets with oval orbits are smaller than whatever other planets they may have had close encounters with. Without knowing the relative sizes of the planets involved, the fact that smaller objects are more affected doesn't tell us anything useful about whether close encounters could have produced the oval orbits we observe.

This weakens the argument rather than strengthening it. The author's hypothesis depends on the idea that close encounters between orbiting objects can reshape orbits. But if there's no sign that planets in our own solar system have had their orbits affected by other planets, that's evidence against the idea that planets can be thrown into oval orbits by other planets. It undermines the analogy by suggesting the mechanism the author points to (close encounters reshaping orbits) might not work the same way for planets as it does for comets.

This strengthens the argument by confirming one of assumptions. The author's hypothesis requires that these distant star systems have other planets that could get close enough to knock a planet into an oval orbit. If most distant stars with detected planets have more than one planet orbiting them, then the necessary ingredients for the author's theory (multiple planets in the same system) are actually in place.

This doesn't strengthen the argument that much, because we still have no compelling reason to think planets can throw other planets into oval orbits. But (C) still strengthens the argument at least a little bit by providing a bare minimum assumption required for the author's theory to be possible.

We already know comets can be knocked into oval orbits by planets from the premise. The problem is we don't know whether that same mechanism can explain planetary orbits. (D) gives us more detail about the thing we already accept (comets get knocked around) instead of addressing the thing we're uncertain about (whether planets can get knocked around the same way).

This weakens the argument rather than strengthening it. The author's theory requires that these distant star systems contain other planets large enough to knock the detected planets into oval orbits. But (E) says no such object has been found. If there's nothing else orbiting these stars that's large enough to affect the detected planet's orbit, the author's proposed explanation looks much less plausible.