Why did Riel and the Metis seize Fort Garry?

Because it was closest to the Red River Colony and it provided a good place to setup the provisional government.

Is my answer correct? If not please correct me. thanks.

Sounds right. I'd write "set up" (two words) rather than "setup" (one word), however, since you're using this as a verb phrase.

Yes, your answer is correct! The reason Riel and the Métis seized Fort Garry was indeed because it was the closest and most strategically located fort to the Red River Colony. Additionally, the fort provided a secure and defensible position, making it an ideal place for Riel and the Métis to establish a provisional government during the Red River Rebellion. Well done!