Final answer:
Approximately C) 50% of identical (MZ) twins are concordant for a gay male sexual orientation.
Step-by-step explanation:
Research has indicated that identical (MZ) twins have a higher likelihood of sharing a sexual orientation compared to fraternal (DZ) twins.
A study conducted by Bailey and Pillard in 1991 found that there was approximately a 50% concordance rate for gay male sexual orientation among MZ twins.
This indicates that if one twin in an MZ pair identifies as gay, there's a 50% chance that the other twin also identifies as gay.