The correct answer is choice C. Three. The normal number of vessels in the umbilical cord is three: one umbilical vein and two umbilical arteries. The umbilical vein carries oxygenated blood and nutrients from the placenta to the fetus, while the umbilical arteries carry deoxygenated blood and waste products from the fetus to the placenta.

Choice A is wrong because one vessel in the umbilical cord is not enough to support the fetal circulation. Choice B is wrong because two vessels in the umbilical cord is a condition called two-vessel cord or single umbilical artery, which can be associated with some birth defects or growth problems. Choice D is wrong because four vessels in the umbilical cord is not a normal variation and can cause complications such as cord prolapse or knotting. The normal range of the umbilical cord length is 50-60 cm and the diameter is 1-2 cm.