Choice "D" is correct. The risk of assessing control risk too high is the risk that the assessed level of control risk based on the sample is greater than the true risk based on the actual operating effectiveness of the control.
Choice "c" is incorrect. The risk of assessing control risk too low is the risk that the assessed level of control risk based on the sample is less than the true risk based on the actual operating effectiveness of the control.
Choice "b" is incorrect. Assessing control risk too high relates to an incorrect evaluation of risk by the auditor, not to whether the control activity relates to the client's assertions.
Choice "a" is incorrect. If the auditor believes that a control activity will reduce the extent of substantive testing when, in fact, it will not, this implies that the control risk based on the auditor's sample was less than the true risk based on the actual operating effectiveness of the control. This would be an example of assessing control risk too low, not too high.