I believe the correct answers are B, C & D as these reflect the three main ways in which GDP is calculated.
1. GDP by production
The value of goods & services or economic output is calculated (Answer B)
2. GDP by income
This is a tally of the national income, which is made up by producers selling things, and the government revenue from taxes (Answer D)
3. GDP by spending
This is the most common method and is a tally of money spent. This includes consumption, investment, government expenses and net exports (answer C)