Overall, I have always maintained a good standing in all of my undergrad classes. Most of my grades are either As or Bs. With only one Junior level course with a D. Is it worthwhile to explain this one bad grade? I am afraid that since all my other grades are good, I will only be highlighting that one bad grade by talking about it.
There are really two parallel circumstances to consider here:
- The grade is highly relevant to the program in question. In that case, you might need to address it, as someone will be focusing on that coursework in particular - partially because they will be relying on you knowing the content of that coursework, and with that grade it's hard to argue you do. Though in this case, it's also probably a good idea to strongly consider retaking the course. In this case, you might touch on the reasons, but that should be 1) Short and 2) Be a legitimate reason why this might not accurately reflect your abilities. "I didn't like the professor" is not that reason.
- It's just a random class. 'Women in Medieval Art and Literature' and you're applying to a Microbiology program. 'Calculus 1' and you're applying to a Comparative Literature program. 'German 4' and you're applying for a Computer Science program. In this case, I wouldn't draw attention to it.