Astudy of the women's movement in the united states would show that
a. the national government granted rights to women long before state governments did.
b. the gains made by women usually took considerable periods of time.
c. women received voting rights before african americans did.
d. wartime employment slowed progress toward gender equality.

Option b: he gains made by women usually took considerable periods of time.

Explanation: In a study, you would see that the fights for the women's rights are usually really long and in a lot of times are dismissed for long periods of time, until their voice is finally heard (even in the actuality you can see that the movement is still asking for rights, especially in countries less advanced, like in South America).  

So the correct answer seems to be option b, and its correct not only because it's true, you will see it in a study because this is and was a really big problem for feminism.