In this reaction you will make predominantly one diastereomer of product in favor of the other. The idea of the question is to draw a transition state which explains why the one diastereomer is preferred.
Here is a hint: the diastereomers would arise from attack at one of the two faces of the cabonyl carbon. How do you think the presence of the t-butyl group will affect these transition states? It might not be an obvious steric interaction, but something which is a consequence of the arrangement of the t-butyl group in space (axial or equatorial). Think about how that affects other substituents around the ring as well.