I want to say D is the answer.
Zygote = sperm + egg, later becomes human
stem cell = differentiates
oocyte = isn't this egg? if so, it can combine with sperm and continue
red blood cell = i don't know if it can develop any further. All i know is what it does (ex> it has hemoglobin to transport oxygen and CO2) and I know that reticulocyte or whatever becomes RBC but I never heard of RBC becoming something.
So I want to say D is the answer.