A good rule of thumb is if you don't know what it means - then its almost certainly "All investment is at risk" (box a)
These rules can be complicated in practice, but they really don't apply to most normal taxpayers. Basically, if your business were to fail or lose money and you would personally lose money, then your investment is at-risk.
If you would not personally lose any money even if your business failed because someone else is guaranteeing your investment or through a non-recourse loan, then your investment is not at-risk. The reason you need to make this distinction is that the IRS will not allow you to take a loss on your Schedule C if you are not at-risk of actually losing any money.
See this TurboTax article for further explanation -