Ome modern eukaryotes, such as giardia intestinalis, lack mitochondria but still retain some of the genes for mitochondrial proteins. what logical conclusion can be drawn about these organisms?

Best answer is: d) At one point during their evolution, these organisms contained mitochondria.

The presence of genes for mitochondrial proteins suggests that these genes are a relic of the time when those bacteria contained mitochondria. Over time and many mutations, the bacteria may have lost the mitochondria, but not necessary purged every gene related to mitochondria. 

A. radioactive dating