I really don't know if people would get the marks for saying motivational theories. It depends on how you worded your answer. Because, IMO.. it wouldn't be appropriate for a HRM to implement a motivational theory on a workforce that it is going to be all out of a job within the near future - the main objective of the theory is to actually motivate the workers so that they put effort into their job, leading to a better organisation.. but as the case study said, the whole plant was 'closing down' - which means it would not be operational and hence, there would be no more workers there to drive productivity.. so to me, it would be pointless using a theory because of the fact the organisation is closing down.
I think it would have been better to say something about helping the employees have a termination that goes smoothly without any high dramas or issues - and the best strategy would be to interview the employees, provide them with interview/resume skills, write them references and organise outplacement programs so they can easily find a new job at the other plants, or at another organisation.
Like I said, you may get marks for saying a motivational theory - but I don't think it is the best option at all.