Simulation

a. does not guarantee optimality.
b. is flexible and does not require the assumptions of theoretical models.
c. allows testing of the system without affecting the real system.
d. all of the alternatives are correct.

d. All of the alternatives are correct.

Explanation:

             This process is usually used to study, predict or analyze the various behaviors exhibited by the real system.

             The simulation process aims at creating a mathematical or logical model that behaves similar to the real system.  

              It tests the real system without bringing any hindrance to its functionality and does not consider any assumptions of theoretical models.

             The simulation process is very flexible but does not guarantee optimality.

Therefor all the given option are correct.

Option d is correct.