Question

similarity
the blue - ringed octopus, puffer fish, and poison arrow frog all have the same type of poison in their saliva.
difference
?
the puffer fish and poison arrow frog use the venom for defense, while the blue - ringed octopus kills its prey with the venom.
the puffer fish and poison arrow frog attack other animals, while the blue - ringed octopus paralyzes scientists with its venom.
the puffer fish and poison arrow frog are never attacked, while the blue - ringed octopus is eaten by other animals for its venom.
the puffer fish and poison arrow frog use the venom to catch prey, while the blue - ringed octopus attracts a mate with its venom.

Explanation:

To find the difference, we analyze the options. The similarity is about poison type, so difference should be about how they use venom. The first left option says puffer fish and poison arrow frog use venom for defense, blue - ringed octopus for killing prey. Let's check other options: second left is about attacking/paralyzing (not difference in use), third is about being attacked (not venom use), fourth is about catching/mating (incorrect). So the first left option (top - most left) explains the difference in venom usage: puffer fish and poison arrow frog use venom for defense, blue - ringed octopus uses it to kill prey.

Answer:

The top - most left text box (with text "The puffer fish and poison arrow frog use the venom for defense, while the blue - ringed octopus kills its prey with the venom.")