First Principles Thinking: classification rule
A is correct. The default-risk-free category in the module is tied to Treasury instruments from highly creditworthy sovereign issuers. Item I, Treasury bills, qualifies as a Treasury instrument even though it is short term. Item II, leveraged loans, appears in the high-yield area. Item III, mortgage-backed securities, appears under investment-grade long-term instruments, not default-risk-free. One item qualifies, so the answer is Only one.
Option B is incorrect. Only two would require either leveraged loans or mortgage-backed securities to be default-risk-free, but the module places them in other credit categories.
Option C is incorrect. All three fails because leveraged loans and mortgage-backed securities are not in the default-risk-free category.