Final answer:
The defense mechanism involving a "self-justifying explanation" in place of the real, less acceptable reason is called rationalization. It allows individuals to justify behaviors with logical reasons, avoiding true reasons that may be discomforting.
Step-by-step explanation:
The defense mechanism that involves a "self-justifying explanation" that is substituted for an acceptable one is known as rationalization. This psychological concept refers to the process where an individual justifies behaviors or feelings with seemingly logical reasons, often avoiding the true reasons which may be less acceptable or more uncomfortable to acknowledge.
Rationalization allows individuals to explain or excuse their behavior by creating a false but plausible excuse. For example, if a student fails a test and then justifies it by saying the test was unfair, rather than acknowledging a lack of study, they are employing rationalization as a defense mechanism.