In one study of almost 4,000 pairs of twins in finland, it was found that genetic factors account for about 40 percent of male twins' homosexual behavior. the results of this study suggest that factors contribute to sexual orientation

In one study of almost 4,000 pairs of twins in Finland, it was found that genetic factors account for about 40 percent of male twins homosexual behavior. The results of this study suggest that biological factors contribute to sexual orientation.
Biological factors that influence sexual orientation includes specific genetic profiles and prenatal hormones. 

It's most likely choice a, it makes the most sense in my opinion.