The answer is option 'A' - Chinese immigrants.

Chinese immigrants played a significant role in providing cheap labor in Australia during the late 19th and early 20th centuries. This answer can be understood by examining the historical context of Chinese immigration to Australia and their involvement in various industries.

1. Chinese Immigration to Australia:
- Chinese immigration to Australia began during the mid-19th century, primarily driven by the Australian gold rushes.
- Thousands of Chinese immigrants migrated to Australia in search of better economic opportunities and the chance to strike gold.
- The Chinese population in Australia grew rapidly, particularly in areas such as Victoria and New South Wales.

2. Economic Contribution:
- Chinese immigrants played a crucial role in various industries, including mining, agriculture, and construction.
- In the goldfields, Chinese miners were known for their hard work and perseverance, often working in harsh conditions and in areas where others had given up.
- They were willing to work for lower wages compared to their non-Chinese counterparts, making them attractive to employers looking for cheap labor.

3. Discrimination and Exploitation:
- Despite their contributions, Chinese immigrants faced significant discrimination and prejudice from both the government and the general population.
- Anti-Chinese sentiment led to the introduction of various discriminatory policies, such as the Chinese Immigration Act of 1881 and the White Australia Policy.
- These policies aimed to restrict Chinese immigration and limit their rights and opportunities.

4. Role in Infrastructure Development:
- Chinese immigrants also played a crucial role in infrastructure development, particularly in the construction of railways and roads.
- They were often employed as cheap laborers, working long hours for low wages.
- Their contribution was vital in connecting different parts of Australia and facilitating economic growth.

5. Legacy:
- The labor provided by Chinese immigrants helped to fuel Australia's economic development during this period.
- Their contributions were often overlooked or undervalued at the time due to discrimination and prejudice.
- However, their legacy is now recognized, and they are seen as an integral part of Australia's multicultural history.

In conclusion, Chinese immigrants in Australia provided cheap labor and made significant contributions to the country's economy during the late 19th and early 20th centuries. Their hard work and perseverance helped to shape the development of various industries and infrastructure projects in Australia.