Explanation
According to the CISM Review Manual, performing a risk assessment is the most important course of action for an information security manager during the due diligence phase of an acquisition, as it helps to identify and evaluate the potential threats, vulnerabilities and impacts that may affect the information assets of the target organization. A risk assessment also provides the basis for performing a gap analysis, reviewing the information security policies and awareness, and developing a remediation plan.
References = CISM Review Manual, 27th Edition, Chapter 3, Section 3.4.1, page 1411.