Final answer:
The largest city in Upper Canada was Toronto, which surpassed other cities in the region such as Kingston and Hamilton in size and importance during the time of the British colony now known as southern Ontario.
Step-by-step explanation:
The largest city in Upper Canada during its time was Toronto. Upper Canada, which existed from 1791 to 1841, was a British colony located roughly in what is now the southern portion of the province of Ontario in Canada. Considering the options given: Hamilton, Kingston, Ottawa, Toronto, and Montreal - Toronto would be the correct answer. It is important to note that Montreal is not located in what was known as Upper Canada, but rather in Lower Canada, which corresponds to parts of the modern-day province of Quebec.
To emphasize, Ottawa was not particularly significant in Upper Canada at that time and later rose to prominence as the capital of the country. Kingston was an important military and economic center, but it was not as populous as Toronto. Hamilton was smaller compared to Toronto during that era.