Which description best defines blastocyst?

a.
a fertilized egg
b.
an egg released from the ovary
c.
a mass of specialized cells
d.
an embryo with a notochord

The correct option is C

Explanation:

A blastocyst consists of a mass of specialized cells (trophoblast), which is formed at the early stage during the development of the embryo. This means the embryo has not been formed yet (an embryo is formed when the amniotic sac develops).

An egg released from the ovary is wrong because it is a single cell (that has nothing to do with pregnancy on its own) while "A fertilized egg" is wrong because a fertilized egg consists of just two cells; a sperm cell and an egg cell".