The structure of texas's highest appellate courts is unique, as only texas and oklahoma organize their courts in this manner. what makes the structure so unusual?

While each state in the United States has one Supreme Court, Oklahoma and Texas have two, one for civil appeals and one for criminal appeals.

The courts are composed by associate justices. In Oklahoma there are nine associate justices  appointed by the Governor through a nominating comission for a one year term, initially, and, if retained, the justice associate is able to assume a six year term. In Texas, there are nine associate justices as well, but nominated through a partisan election for a six year term.

the 2nd option is the answer