Armature reaction of a synchronous generator
When the synchoronous generator loaded, an angle δ occurs between stator magnetic field and rotor magnetic field. It is called "load angle". Hence the IA represents the stator magnetic fied and the BS is now perpendicular to this surface. Now the terminal voltage VΦ is related to Bnet, Bnet=BS+Br. You can observe exact load angle δ between VΦ and EA in your figure (d). Also the load angle δ is shown on voltage vector diagram between Ea and VΦ.
Sorry my poor English. I hope i can explain it.