ExplanationGiven the clue most people already know, you know that the way the Secret Service employees are described in the first part of the sentence—in this case sharp-eyed—is the same as the way they are described in the second part. Thus, vigilant and mindful are appropriate for the blank.
Robust and potent are a synonym pair which means strong, which is not supported by the clue, although they may indeed be strong people. They may also be vulnerable in some ways, but this choice would be the opposite of the intended meaning. The same applies for weary, which means tired.
Hence options E and F are correct!
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