This one.
There is one study that controlled for same-sex attraction, at least in transwomen, and if I recall correctly, that study found that transwomen who were attracted to men, did have somewhat feminized brains compared to men. Transwomen who were attracted to women had changes in the size of various brain regions, but not in such a way that their brains were feminized, and not in the same regions as male-attracted transwomen.
I don't think anyone knows exactly what these differences mean, or why they manifest as a desire to wear women's clothes rather than, as, say, a desire to take care of people.
I think it would be interesting to understand the brain basis of dysphoria with one's body. I would have predicted a common region of the brain being affected as compared to people who have dysphoria with non-gendered body parts. But I don't know if anyone is doing that research.
Again, from a societal viewpoint, it sorta doesn't make sense to use brain differences to justify mixing the resources set aside for women, because we have different resources for men and women, not because of different brains, but because of being born with different bodies.