Frank was … in the riot because his books were found at the scene.

Analyze the question: The sentence says Frank's books were found at the scene of the riot. This is evidence suggesting he was involved, so we need a word meaning "shown to be connected with a crime or wrongdoing."

Explain the concept: To implicate someone is to show or suggest that the person is involved in a crime or wrongdoing. It is usually followed by "in", as in "implicated in the riot."

Check each option:

• A. balmed: Not a standard English word in this sense. "Balmed" would relate to applying balm (an ointment) and makes no sense here.
• B. caught: "Caught in the riot" could mean he was trapped in it. But the clue about his books being evidence points to suspicion of involvement, not just being present. "Caught" also does not fit the idea of evidence pointing to him.
• C. implicated: Fits perfectly. His books at the scene suggest he took part, so he was implicated in the riot.
• D. grafted: This means joining plant tissue, or working hard (in slang). It does not fit.

Answer: The correct option is C. implicated.

Tip: When you see the preposition "in" with evidence-related clues (found at the scene, fingerprints, documents), think of implicated in. Remember: implicate = involve in wrongdoing. Its opposite is exonerate (clear of blame).