Why D? And why not the others?

Correct answer: D. The African elephant

African elephants gestate for around 22 months — the longest pregnancy of any mammal — and give birth to calves already weighing roughly 100 kilograms. The marathon gestation is thought to give the calf's large brain time to develop, so a newborn can stand within minutes and walk with the herd within hours. The blue whale seems the obvious answer because it is by far the biggest mammal, yet its pregnancy lasts only about 11 to 12 months — a calf that grew inside any longer would become impossibly large to birth and nurse at sea. The giraffe is a strong near-miss at roughly 15 months, and the sperm whale, at around 14 to 16 months, has one of the longest gestations among whales — but the elephant outlasts them all. Bigger body does not always mean longer wait.

Source: Encyclopaedia Britannica: elephant — reproduction and life cycle (gestation 18-22 months, longest of any mammal)