Answer:
When dealing with an interview question, it is always the best policy to be as open and honest as it is possible, but without having to divulge information that is not absolutely relevant to the conversation, or to the purpose of the interview.
In this type of question: Tell me about a time when you had to leave a task unfinished, there are two very important things to keep in mind. The first, be honest, and the second, be precise and concise with the information and provide the necessary explanation as to why you had to do something like that. Honesty, openness, but also, clarity on there being a good reason for not having finished a task, will look interesting to interviewers.
So, if I had to answer a question like this, I would say something like this: Usually I tend to finish all of my tasks, that is why I try to avoid multitasking if possible, because it is better to finish a few thngs, than start many and leave them undone, or halfway. But, when I was in college, I was requested by my team members at a project from school, to finish the project. I agreed, because I like helping out as much as possible, but at that moment, I received a call from my mother telling me that I was needed to take care of my grandfather. So, of course, I called my team members, explained why I couldn´t do what was asked, but I left all the channels of communiation open so that I could help out with what was needed to finish the project, while at the same time helping out with my grandpa.