Leng Bo, CFA is a bond portfolio manager for individual investors. Last year, a client whose portfolio is limited to investment-grade bonds approved Bo's purchase of a below investment grade bond. Because yields in the high grade fixed income markets declined, Bo subsequently decides to enhance this client's portfolio by investing in several additional bonds with ratings one or two notches below investment grade. The investment strategy implemented by Bo most likely violated which of the following CFA Institute Standards of Professional Conduct?
A is correct because the client only approved the purchase of one below investment grade bond while the portfolio manager has purchased several additional bonds below investment grade without client approval in violation of Standard Ⅲ(C).