Answer:
When two different sperm fertilize two eggs, fraternal twins result (option a).
Explanation:
Fraternal twins are the result of the separate fertilization of two maternal eggs by two paternal sperm, with the obtention of two zygotes with different DNA.
Although the embryonic development of fraternal twins occurs at the same time in the maternal uterus, each embryo will have an individual placenta and embryonic sac, with different development and characteristics, sometimes including sex.
The other options are not correct because:
b. Identical twins come from the division of the zygote after fertilization, obtaining siblings with the same DNA and who share the same placenta and embryonic sac.
c. Twins of a different sex do not always occur.
d. Same sex twins always occur when they are identical twins.
