Marginal costing principles in this question is another word for key factor analysis (which you should have studied for F2 and F5 or their equivalents under the old syllabus).
If you are ranking products when there is only limited time available, you rank them on the basis of the contribution per hour (i.e. the contribution per unit divided by the hours per unit)
Have a look at Chapter 5 of the Paper F5 Course Notes on here – it should not take you long to pick it up. (I have a lecture on it on here also if you have a problem with it)