Mammals reproduce through a process called sexual reproduction. during this process, which involves two a.
a female egg cell and a male sperm cell each develop separately into an offspring that is different from both parents.
b.
a female egg cell combines with a male sperm cell to make an offspring that is identical to one of the parents.
c.
a female egg cell and a male sperm cell each develop separately into an offspring that is identical to its parent.
d.
a female egg cell combines with a male sperm cell to make an offspring that is different from both parents.
parents

With sexual reproduction a sperm coming from the male must unite with the female egg. Both are needed for this kind of reproduction, and an offspring CAN'T  be formed without both. Unlike asexual reproduction, the offspring of a mammal reproducing sexually is DIFFERENT from both parents, since the offspring is a combination of both the mother and the father.

An easy to remember example I use when thinking about what sexual reproduction is:

the formation of a human child. A mother with her egg and the father with the sperm is BOTH needed. If you think about siblings that come from the same parents (and therefore same gene pool) they are not all exactly identical to each other. Although they may have similar features, they look different and are NOT genetically identical

Therefor for your question the answer is D.

Hope this helped!

D. a female egg cell combines with a male sperm cell to make an offspring that is different from both parents

Explanation:

if your own plato the answer is b. the fern transitioned from sporophyte generation to gametophyte generation.