It means justify/give reasons for your answers (or, as you put it, "make your answers more understandable") - it is an unusual use of "motivate" which doesn't sound great to me (so to that extent, I agree with the other people who have answered) but I have seen it used this way in exam questions in the UK more than once. Investigating further, it looks like this use of "motivate" may have come from South African English - https://teachenglishtoday.org/index.php/2010/06/a-motivation-against-motivate/ is an article by a South African professor, who doesn't really like it either but explains where it probably originated.

I agree with Derek. I don't think that 'motivate' is the right word here. You can motivate a person to do something, but I don't see how you can motivate answers. It doesn't make sense.

I think a better option would be to "justify your answers" - in other words, don't just write down the answer, but also show the working (in maths, for example). Personally, I'm not 100'% convinced that 'motivate' is the correct word here.