Vera: One needs to be familiar with the two compounds in terms of their structures and functions. Monensin is an ionophore forming complexes with monovalent cations such as Na+, K+, etc, and transporting them across lipid membranes in an electroneutral fashion. Brefeldin A is a different fungal compound that can disrupt the Golgi structure and effectively stop protein traffic/post-translational processing from ER to Golgi. So, you can decide which one to use to answer what question. Hope it helps.