The correct answer is D. Class is being attended.
A PERT event is a point in time at which a task or activity is completed. It is a milestone that marks the end of one task and the beginning of another.
A. Site investigation started is a PERT event because it marks the beginning of the site investigation task.
B. Sessional work completed is a PERT event because it marks the completion of the sessional work task.
C. Bus starts from Jaipur is a PERT event because it marks the beginning of the bus journey from Jaipur.
D. Class is being attended is not a PERT event because it is not a point in time at which a task or activity is completed. It is a description of an ongoing activity.