a)
Network diagram is following:
b)
Gantt chart based on early start time is following:
Resource histogram is following:
In the above resource histogram, we see that there is resource overload in periods 3 to 9
This resource overload can be removed by rescheduling activity C to start from period 9
Resulting Gantt chart is following:
Resource histogram is following:
We see that project completion time remains unchanged before and after resource leveling.
Project completion time = 30