Answer:
d. A risk-free arbitrage opportunity exists.
Step-by-step explanation:
A zero-investment portfolio refers to a portfolio of assets which all the investments its contains has a collective net value that is zero which is obtained when there is a simultaneously purchasing securities and selling equivalent securities.
Alpha refers to the active return on an investment. This implies that when a zero-investment portfolio has a positive alpha, its active return is positive. This can only occur when there is a risk-free arbitrage opportunity.
A risk-free arbitrage refers to the opportunity to make some profit with no capital investment.
Therefore, the correct option is d. A risk-free arbitrage opportunity exists.