It was actually the 26th Amendment, ratified in 1971, that lowered the legal voting age from 21 to 18. The main reason why the voting age was lowered is because men were being drafted to serve in World War II and the Vietnam War at the age of 18. In the 1970 Supreme Court case Oregon vs. Mitchell, judges ruled that Congress had the constitutional right to adjust the federal voting age but not the local or state voting ages. The ruling led Congress to promptly add the 26th Amendment in 1971, which President Nixon speedily signed into law in honor of the troops being sent to Vietnam.
Hence, we can use process of elimination to discard answers A, D, and E from your answer choices since we know the 26th Amendment, added in 1971, has to do with voting age, not the much earlier Voting Rights Act of 1965.
After the Civil War, Congress ratified the 15th Amendment in 1870 to enfranchise newly freed African Americans. The 15th Amendment guaranteed that no American would be denied the right to vote "on account of race, color, or previous condition of servitude" (as cited in Encyclopaedia Britannica, "Fifteenth Amendment"). However, during the days of Reconstruction, Southern states used fraudulent means to disenfranchise African Americans. Such means included forcing voters to take literacy tests, pay poll taxes, and restricting voting rights to property owners. Some states even employed the infamous grandfather clause, which made voters exempt from having to take literacy tests, pay poll taxes, or own property if the voter freely enjoyed voting rights prior to 1866 or 1867. As a result of Southern states' corrupt efforts, nearly half of all African Americans were disenfranchised by the 20th century.
Throughout the 1940s and early 60s, the Supreme Court and Congress took measures to try and enfranchise more African Americans; however, such measures were generally unsuccessful. It was not until John F. Kennedy signed the Civil Rights Act of 1964 and ratified the 24th Amendment and Lyndon B. Johnson signed the Voting Rights Act of 1965 that all African Americans became, once again, fully enfranchised.
Hence, since these measures, including the Voting Rights Act of 1965, were enacted to undo the methods of disenfranchisement Southern states had unconstitutionally employed, we can say that the Voting Rights Act of 1965 represents a federal attempt to control states, which is answer C.