Complete the intergalactic zoologist's field notes about a newly discovered planet by choosing the correct phrase for each blank.

During the expedition, we encountered five distinct alien species, some of whose defense mechanisms included emitting a scent like burnt toast. We also collected hundreds of peculiar glowing rocks, several of which began to hum softly in the cargo bay.

During the expedition, we encountered five distinct alien species, some of whose defense mechanisms included emitting a scent like burnt toast.

We need a possessive relative pronoun here because the "defense mechanisms" belong to the alien species. "Some of whose" perfectly bridges the gap. "Some of which" would be grammatically broken here, and "some of them" creates a comma splice.

We also collected hundreds of peculiar glowing rocks, several of which began to hum softly in the cargo bay.

When referring to inanimate objects (rocks) in a non-defining relative clause, we use "which." "Whom" is reserved exclusively for people (or highly personified beings), and "many of them" would result in a comma splice because there is no coordinating conjunction.