Final answer:
The error where a biometric system falsely rejects a valid user is called a Type 1 error. This occurs when a correct null hypothesis is mistakenly rejected.
Step-by-step explanation:
The error described in the question where a biometric system falsely rejects a valid user is known as a Type 1 error. This type of error occurs when the decision is made to reject the null hypothesis when, in fact, the null hypothesis is true. In the context of a biometric system, the null hypothesis usually states that the person is a valid user; hence rejecting this true claim results in a Type 1 error.
A Type 2 error, on the other hand, would be not rejecting the null hypothesis when it is actually false. The third and fourth options given, Crossover Error Rate (CER) and Equal Error Rate (EER), refer to thresholds or points where the rates of Type 1 and Type 2 errors are equal in a biometric system, not to the specific errors themselves.