Why C? And why not the others?

Correct answer: C. Three

An octopus has three hearts: one systemic heart that drives blood around the body, plus two branchial hearts that each push blood through a gill to pick up oxygen. The extra pumps exist because octopus blood uses hemocyanin, a copper-based molecule that turns the blood blue and carries oxygen less efficiently than our iron-based hemoglobin, so the circulatory system needs reinforcement. A single heart is the intuitive answer because that is the plan we know from mammals, birds and fish. Two sounds plausible if you picture one pump per gill and nothing else, while four feels like a safe guess for such a famously strange animal — but it overshoots by one. Strangest of all, the main systemic heart stops beating whenever the octopus jets through the water, which is one reason octopuses generally prefer to crawl.

Source: Natural History Museum (London) & Smithsonian Ocean: octopus anatomy — three hearts, blue copper-based blood