The earlier in the process that security is planned for and implement the cheaper it is. It is also much more efficient if security is addressed in each phase of the development cycle rather than an add-on because it gets more complicated to add at the end. If security plan is developed at the beginning it ensures that security won’t be overlooked.
The following answers are incorrect:
The system is optimized prior to the addition of security. Is incorrect because if you wait to implement security after a system is completed the cost of adding security increases dramtically and can become much more complex.
The system is procured off-the-shelf. Is incorrect because it is often difficult to add security to off-the shelf systems.
The system is customized to meet the specific security threat. Is incorrect because this is a distractor. This implies only a single threat.