After twins are conceived, their chance of survival until birth depends on the prenatal circumstances. An early sonogram might reveal two developing organisms, but later only one embryo continues to grow. This is referred to as the twin phenomenon.

vanishing  twin phenomenon

Explanation:

Vanishing twin phenomenon or Vanishing twin syndrome can also e referred to as disappearing twin syndrome. This phenomenon is used to describe the miscarriage or loss of one of two or more developing babies during the first trimester. One of the twins vanishes at the early onset of a multiple pregnancy. During this period, the fetal tissue of the said vanishing twin is absorbed by the other twin, placenta, other fetuses, or the mother.

correct answer is a. and d.