This one was tricky but I think you could’ve done this one without knowledge of NMDA receptors. Stem told you that glutamate activates both non-NMDA and NMDA receptors but it activated only non-NMDA receptors in the early phase. That means NMDA receptors activate after non-NMDA receptors. That means something was delaying NMDA receptor activating and the only answer that made sense as the Mg inhibiting NMDA at resting potential. Once the cell is depolarized by non-NMDA receptors, NMDA receptors can be activated.
don't forget about the Mg block! . . . . . .