To answer this question, we need to understand how JNDI (Java Naming and Directory Interface) declarations work for JDBC resource references.
JNDI is a naming service that allows Java software components to discover and access named resources. In the context of a Java EE application, JNDI is used to define and access resources such as databases.
In this question, the resource reference is named "enrolment" and it is a JDBC resource reference for a DB2 database. The appropriate JNDI declaration for this resource reference would be:
B. java:comp/env/jdbc/enrolment
The JNDI declaration consists of a hierarchical naming structure, starting with "java:comp/env" which refers to the "environment" namespace of the component. The "jdbc" component is then specified, followed by the name of the resource reference, which in this case is "enrolment".
Option A) java:comp/env/jdbc/db2/enrolment - This option is incorrect because it includes an additional "db2" component in the naming structure, which is not specified in the question.
Option C) java:comp/env/enrolment - This option is incorrect because it does not include the "jdbc" component in the naming structure, which is necessary for a JDBC resource reference.
Option D) java:jdbc/env/db2/enrolment - This option is incorrect because it includes "jdbc" and "db2" components in the naming structure, which are not specified in the question.
Therefore, the correct answer is B. java:comp/env/jdbc/enrolment. This option is correct because it follows the appropriate JNDI declaration for a JDBC resource reference named "enrolment".