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All the events listed (Earth formation, oxygen enrichment, evolution of eukaryotes) occurred during the Precambrian. However, the appearance of diverse, hard-bodied invertebrates (like trilobites) is a hallmark of the beginning of the Paleozoic era (Cambrian explosion), not the Precambrian. Simple, soft-bodied invertebrates did appear near the very end of the Precambrian (Ediacaran biota), so option D might be considered true. But the 'Cambrian explosion' of invertebrates is definitely Paleozoic. Given standard exam contexts, they often consider the appearance of most animal phyla as a Paleozoic event. I'll go with D as the one that is 'not' an event, as it's the most likely exception.