Final answer:
The significant increase in population prior to Confederation in 1858 was primarily due to the Gold Rush, as it attracted many people seeking to make their fortunes in mining for gold.
Step-by-step explanation:
Primary Reason for Population Increase Prior to Confederation in 1858
The large increase in population in the area that would become Canada prior to Confederation in 1858 was primarily due to the Gold Rush. This momentous event saw a significant influx of people, hoping to make their fortunes by mining for gold. The alternative options you provided, such as 'increased number of jobs in mining', 'employment in Athabasca tar sands', and 'because of the Canadian Pacific Railway', do not apply to the 1858 context as they either occurred later in history or, in the case of the Athabasca tar sands, were not a significant employment sector at that time.
While mining itself did generate job opportunities, it was the Gold Rush that triggered the initial population surge, rather than the general availability of mining jobs. Also, the Canadian Pacific Railway was not constructed until the 1880s, which is well after the 1858 period in question. Similarly, the development of the Athabasca tar sands as an employment sector came much later in Canadian history.
Answer: a. Due to the Gold Rush