The effectiveness of a control has decreased.

 The most likely effect on the associated risk when the effectiveness of a control has decreased is that the residual risk changes. Residual risk is the risk that remains after the implementation of risk responses or controls. If the control becomes less effective, the residual risk will increase, as the risk exposure and impact will be higher than expected. The risk impact, the risk classification, and the inherent risk are not likely to change when the effectiveness of a control has decreased, as they are more related to the nature and characteristics of the risk, rather than the control performance. References = Risk and Information Systems Control Study Manual, 7th Edition, Chapter 2, Section 2.1.1.4, page 541

1: ISACA Certified in Risk and Information Systems Control (CRISC®) Exam Guide, Answer to Question 652.