Why D? And why not the others?

Correct answer: D. Ottawa

Ottawa was hand-picked by Queen Victoria in 1857, an obscure lumber town that had been known as Bytown until its 1855 renaming. Toronto, Montreal, Quebec City and Kingston were locked in such fierce competition to host the government that the choice was referred to the queen, who settled on Ottawa partly because it sat right on the boundary between English-speaking Canada West and French-speaking Canada East - a diplomatic middle ground - and was safely inland from the American border. Toronto lures answerers as Canada's largest city, Montreal was long its commercial metropolis and briefly a seat of government, and Vancouver is merely the Pacific coast's biggest city; none holds the capital title.

Source: Britannica: Ottawa (Queen Victoria's 1857 selection)