Final answer:
Rationalization is the defense mechanism that involves a self-justifying explanation substituted for an unacceptable one.
Step-by-step explanation:
The defense mechanism that involves a self-justifying explanation that is substituted for an unacceptable one is rationalization. Rationalization is a process where individuals justify their behaviors by substituting acceptable reasons for less-acceptable real reasons. For example, someone who failed an exam might rationalize by saying that they didn't study enough, instead of admitting their lack of knowledge or ability.