In the context of core training, the core typically includes the abdomen, lower back, glutes, and pelvis. All the options listed are considered part of the core. However, if you need to identify an area that is less often directly referred to as part of the core in some contexts, it might be appropriate to say that the pelvis itself is not always emphasized compared to the others, even though it plays a supportive role.
That said, strictly speaking, none of the listed body parts can be ruled out as part of the core, but if forced to choose one that is sometimes considered less central, you might select "her pelvis."