Answer:
THEIR...
Two days after the aquarium's researchers received THEIR first octopus, the octopus escaped through a grate in the floor and found its way back to the ocean.
Explanation:
The researchers are "them" and the octopus is "it." The possessive of "them" is "their" and the possessive of "it" is "its." Since we are discussing the aquarium's researchers - not its octopus - the correct joining word is "their."
