Could be true. We know that elephant seals can dive to greater depths than Weddell seals can, since elephant seals can stay submerged longer. And, we don't have any facts that allow us to compare dolphins to Weddell seals or elephant seals. So it's possible that elephant seals can dive deeper than dolphins, and that dolphins can dive deeper than Weddell seals.

Could be true. We know that dolphins can dive to greater depths than northern fur seals, which implies that dolphins can stay submerged longer than northern fur seals. And, we don't have any facts that allow us to compare the diving depths of dolphins and Weddell seals. So it's possible that dolphins can dive deeper than Weddell fur seals, and that Weddell fur seals can dive deeper than northern fur seals.

Could be true. We know that dolphins can stay submerged longer than northern fur seals can, because dolphins can dive deeper. We don't know how Weddell seals compare to dolphins or to northern fur seals. So it's possible that Weddell seals can dive deeper than both dolphins and northern fur seals, and therefore can stay submerged longer than both dolphins and northern fur seals.

Must be false. We know that dolphins can dive deeper and stay submerged longer than northern fur seals. We also know that elephant seals can dive deeper and stay submerged longer than Weddell seals. So if northern fur seals can stay submerged longer than elephant seals, as the first part of (D) asserts, then this would imply that northern fur seals can stay submerged longer than Weddell seals (because elephant seals can stay submerged longer than Weddell seals). But now consider the second half of (D), which states that Weddell seals can dive deeper than dolphins can. This would imply that Weddell seals can stay submerged longer than dolphins and northern fur seals, too (since dolphins can stay submerged longer than northern fur seals). This creates a contradiction. The first half of (D) implies that northern fur seals stay submerged longer than Weddell seals. But the second half of (D) implies Weddell seals stay submerged longer than northern fur seals. This is why (D) must be false. If both parts of (D) were true, there's a logical contradiction. So (D) cannot be true.

Could be true. We know that elephant seals can dive deeper than Weddell seals. And, we don't know the comparative diving depths of elephant seals and northern fur seals. So it's possible that elephant seals can dive deeper than northern fur seals, and for northern fur seals to dive deeper than Weddell seals.