Answer:
B. Bisexuality
Step-by-step explanation:
Bisexuality describes a sexual orientation, because refer to people who are sexually attracted to both genders, for example a man who likes sexually women and men.
Transgender isn't a best describer of sexual orientation because specifically refers to people who doesn't field identify with their gender. A transgender person not necessarily is homosexual for example, there are cases where women don't feel themselves as such, but they still like men.