When creating a parallel structure between two nouns, it is NOT necessary for those nouns to match in number. We can compare one dog to multiple cats, we can compare the purple dog to cats (without any modifiers), etc.

For reference, see OG 13/2015 SC #121. The correct answer reads:

New theories propose that catastrophic impacts of asteroids and comets may have caused reversals in the Earth's magnetic field, the onset of ice ages, the splitting apart of continents 80 million years ago, and great volcanic eruptions.

Here, REVERSALS and ERUPTIONS are plural; THE ONSET and THE SPLITTING are singular. ERUPTIONS had the modifiers GREAT VOLCANIC in front of it, but the others didn't have modifiers. None of that matters; as long as we're comparing nouns - nouns that make sense to compare - it's parallel.

In your example, the GMAT is trying to mess with your "ear." It sounds funny to compare other arachnids (plural) to the scorpion (singular), but it's perfectly grammatically correct.

The answer is C, because it's the only one that compares ANIMAL to ANIMAL.