Sign-by-sign elimination:
Goodell's = cervix feels soft (like lips instead of nose tip): describes texture only, not separation, so out.
Chadwick's = bluish-violet colour of vagina and cervix from venous congestion: a colour sign, not a consistency one, so out.
Piskacek's = lop-sided uterine enlargement at the implantation site: asymmetry, not isthmic softening, so out.
Hegar's = the soft, compressible isthmus at 6 to 8 weeks lets the bimanual fingers almost appose, making cervix and body feel disconnected. This is exactly what the stem describes.
Memory aid: 'Hegar = Hinge.' The soft isthmus acts like a hinge between the firm cervix below and the soft fundus above. That image points straight to option d.