In robust optimization, what is meant by the term "hard constraint"?

A constraint that is not violated

A constraint that has a shadow price of zero

A constraint that can be violated slightly without serious repercussions

A constraint that can be violated dramatically without serious repercussions

A constraint that cannot be violated

Mixed problems may have the following type of constraints:

$$

$$

$($c$)=$

All of the answer choices are correct.

None of the answer choices are correct.