Which of the following is a requirement for attorneys handling client funds?

Attorneys have an ethical obligation to handle client funds with the utmost care and integrity. One fundamental requirement is that they must operate a separate trust account specifically for client funds. This separation ensures that client money is not commingled with the attorney's personal or business funds, which protects the client's interests and adheres to the fiduciary duty that an attorney has.

By maintaining a separate trust account, attorneys can accurately track client funds and ensure that they are used solely for their intended purposes, whether for holding retainer fees or funds meant for disbursement. This account must comply with specific regulations to ensure accountability and transparency, which are crucial for maintaining client trust and safeguarding against misappropriation.

In contrast, the other options do not align with the ethical standards set for attorneys. For example, attorneys are not required to disclose all client funds at the beginning of representation, nor are they obligated to invest client funds for profit. Furthermore, borrowing from client funds for personal use is strictly prohibited and considered misconduct. Thus, the requirement to operate a separate trust account for client funds is the most appropriate and essential obligation for attorneys in this context.