Final answer:
An investor cannot attain a portfolio above the feasible or opportunity set due to budget constraints, risk, and return.
Step-by-step explanation:
The reasons why an investor cannot attain a portfolio above the feasible or opportunity set are budget constraints, risk, and return. Budget constraints refer to the limited amount of money available for investments, which can restrict the options and prevent an investor from building a portfolio beyond the feasible set. Risk and return are important factors to consider in investing, as some investments may carry higher risks and lower returns, making it difficult to achieve a portfolio above the opportunity set.