Based on historical perspective, the reason activists demanded federal intervention in Selma is the "Lack of enforcement of voting rights for blacks."
The Selma March occurred on March 7, 1965, where many activists formed to protest the lack of enforcement of voting rights for blacks.
Before the match, there have been rules and constitutional rights that allow black to vote, but despite that, many states in the South still deny many blacks voting rights.
Therefore, the activists at Selma called upon federal intervention to ensure that African Americans could exercise their constitutional right to vote.
Hence, in this case, it is concluded that the correct answer is "Lack of enforcement of voting rights for blacks."
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