@domt73 parent opinion here. You have nothing to explain. Getting one B+ or even a C isn’t the end of the world.
This just is worth one more minute of worry on your part. Even if it’s the only not A grade you have when you are applying for colleges in a couple of years, I would still suggest not providing any explanation for a B+ grade.
I’m sorry you are dealing with your parent divorce, but really…your grade of B+ in one class…just put that out of your mind.