Auto focus mode

Hello guys, I got my Nikon D750 not too long ago & this is my 1st post. I have a question on what's the ideal/best auto focus mode for taking portrait with a very shallow depth of field setting (like f1.8/2.8)? Like after I have focused on the subject's eye and then recompose, the eyes should still remain sharp even if me or the subject have move a bit (forward & sideways). My guess is AFA with 3D tracking, am I right? Or any other suggestions? Thanks in advance.

Hi jazz
i have the 7100 and just started using Back Button focusing which I'm pretty sure
locks focus until pressed again think they call it AF ON
there will be loads of help along soon this is a great site if you got a prob
good luck