Irrelevant. The reasoning isn't based on the precise age of the sandstone, just on the fact that it was made millions of years before the earliest traces of multicellular animal life.

This strengthens the author’s argument. If geological processes are known to have left a variety of marks in sandstone from around this period, then it seems more likely that those processes could explain these marks, too.

This doesn't weaken the argument. The idea that other organisms made these worm-like tracks doesn't weaken the reasoning, since the argument is based on the claim that these marks were created millions of years before multicellular life is known to have existed, not just before worms existed. It would weaken the argument if we knew that multicellular life in general possibly emerged long before the earliest surviving traces--but this answer choice doesn't tell us that.

This rules out geological processes as an explanation for these marks. Thus, though this answer choice doesn't provide a specific alternative explanation, or open up the possibility that multicellular life might have existed much earlier than the earliest surviving traces, it significantly weakens the argument and points to some other cause for the tracks.

This answer choice is tricky. Notice that it focuses on the emergence of worms, and not on the emergence of multicellular life in general. To weaken the argument, this answer choice would have to tell us that multicellular life in general emerged much earlier than previously thought and left no traces, but this answer choice doesn't say that. It just tells us that once multicellular life actually did emerge, worms were among the first forms of such life to emerge. So this doesn't harm the author's assumption that the earliest known traces of multicellular life give us a rough date for the actual emergence of multicellular life, and so this answer choice doesn't weaken the argument.