Concept:
Warning colouration in toxic prey works only if predators learn to associate a look with a bad experience. When several toxic species look alike, every attack a predator makes teaches it to avoid that one shared pattern, which protects every species carrying it.
Step 1: Note the two facts given in the question.
Both butterfly species are toxic to the same predator, and they resemble each other closely. Neither species is harmless.
Step 2: Recall what Batesian mimicry needs.
Batesian mimicry needs one species that is actually dangerous or toxic (the model) and a second, harmless species (the mimic) that copies its look to trick predators. Since both species here are toxic, there is no harmless mimic, so this is not Batesian mimicry.
Step 3: Recall what Mullerian mimicry needs.
Mullerian mimicry needs two or more species that are all genuinely defended in some way, converging on a shared warning signal. A predator that samples any one of them learns to avoid the whole group, so the cost of teaching the predator is spread across every species that shares the pattern. This is exactly the situation described.
Step 4: Rule out the last two options.
Parasitism describes one organism harming another it lives on or in, not a shared defense signal. Sensory overload is about flooding a predator's senses, not about shared toxicity and appearance.
Step 5: Final Answer.
The phenomenon described is Mullerian mimicry.
\[ \boxed{\text{Mullerian mimicry}} \]