President Nixon endorsed Vietnamization because
Of the answer that you give, D is the best. B is also somewhat possible, but the fact that it uses the word "immediately" makes it incorrect.
The main point of the Vietnamization strategy was to get US troops out of Vietnam. However, Nixon did not believe that this would be immediately possible. Instead, he believed that he would be able to bring the troops home more quickly. As the link below says,
Under this new policy, American forces would gradually transfer responsibility for Saigon's defense to the South Vietnamese military, freeing U.S. troops to return home.
Note the use of the word "gradually." This shows that Nixon did not expect immediate results.
Since B is not correct, D is the best answer because Nixon did hope that the South Vietnamese government would become stronger and more stable.