The correct answer is alternative C) "Many abandoned their German language and customs".
When German immigrants first got to Texas their language was widely used, mainly in the central part of Texas. It took them years until the language started to fade.
Besides, there used to be German festivals, including Saengerfest and Volkfest, which proves they didn't abandon their customs.
For these reasons, alternative C is the correct one: They didn't abandon their language and customs once they arrived in the US.