Sport is to baseball as a stem cell is to 1) cancer 2) skin cell 3)blastocyst 4) animal cell

3) Blastocyst

As baseball is a type of sport, like wise stem cell is a type of cell which is produced from the Blastocyst of the developing embryo.  

In the 3-to 5-day-old fetus, called a Blastocyst, the internal cells offer ascent to the whole body of the living being, including the majority of the many particular organs and cell types, for example, the heart, skin, lungs, sperm, eggs and different tissues.  

In some grown-up tissues, for example, bone marrow, muscle, and brain, discrete populaces of grown-up undifferentiated organisms produce substitutions for cells that are lost through ordinary mileage, damage, or illness.

what's the rest of the question