The correct statement for thermal and mechanical energy in a control volume $($open system$)$ is

a$.$

the increase in thermal and mechanical energy stored equals the amount generated.

b$.$

the amount of thermal and mechanical energy that enters equals the amount that leaves.

c$.$

the increase in the thermal and mechanical energy stored equals the amount that enters minus the amount that leaves.

d$.$

the increase in thermal and mechanical energy stored equals the amount of thermal and mechanical energy that enters minus the amount of thermal and mechanical energy that leaves plus the amount of thermal and mechanical energy generated.