The BEST way to justify the implementation of a single sign-on (SSO) product is to use:

A.
return on investment (ROI).

B.
a vulnerability assessment.

C.
annual loss expectancy (ALE).

D.
a business case.

Explanation:
A business case shows both direct and indirect benefits, along with the investment required and the expected returns, thus making it useful to present to senior management. Return on investment (ROI) would only provide the costs needed to preclude specific risks, and would not provide other indirect benefits such as process improvement, learning, etc. A vulnerability assessment is more technical in nature and would only identify and assess the vulnerabilities. This would also not provide insights on indirect benefits. Annual loss expectancy (ALE) would not weigh the advantages of implementing single sign-on (SSO) in comparison to the cost of implementation.