The control that causes all log-in accounts to become inaccessible after three incorrect password attempts is primarily aimed at preventing unauthorized access to the system. While this is an important security measure, it is not highly effective in detecting fraudulent loans because it does not directly address the loan approval or disbursement processes. Controls that directly check the authenticity and accuracy of loan approvals and customer information, such as management approvals for high-value loans (Option B) or matching customer information to payment data (Option C), are more effective in detecting fraudulent loans. Similarly, preventing the delegation of approval authority during supervisors' vacations (Option D) ensures that approvals are not made without proper oversight, thus helping to prevent fraud. Therefore, the least effective control in detecting a fraudulent loan is the one involving log-in account accessibility after incorrect password attempts.