Explanation/Reference: https://www.lexology.com/library/detail.aspx?g=79732c5e-b4bf-456a-9604-712f44f5a930

Submitting the concern anonymously to the bank's internal Compliance Hotline is the most appropriate action that a bank CEO's assistant should take when the bank CEO expenses large sums of money to a charitable organization run by the bank CEO's direct family member. This is because this action would allow the assistant to report a potential case of fraud, conflict of interest, or misuse of funds without fear of retaliation or reprisal from the bank CEO. The Compliance Hotline is a confidential and secure channel for employees to raise any concerns or suspicions about unethical or illegal activities within the bank. The Compliance Hotline would then forward the concern to the relevant authorities or departments for further investigation and action.
References: = The main references for this question are the following sources:
The document titled "Anti-Fraud Policy" published by the World Bank Group. You can access it by clicking here. This document states that "The World Bank Group has established a Compliance Hotline to receive reports of suspected fraud or corruption in World Bank Group-financed projects or activities, or in the World Bank Group's corporate procurement or administrative activities. The Compliance Hotline is available to all staff and external parties, such as contractors, consultants, beneficiaries of World Bank Group-financed projects, and the general public.

Explanation
Another simpler way of looking at the problem is by seeing this case as a Correspondent Banking relationship:
-A is the primary correspondent bank.
-B is the respondent bank.
-C is the receiving bank (of the wire transfer).
Since A is conducting the transaction on behalf of B's customer via the correspondent relationship deemed suspicious, it is the transaction in Country A by the correspondent bank that files the STR. Also, per the regular process, it is on the transaction, not the bank, that the STR is filed against.