Step 1: Understanding the Concept:
This question is from the story "A Question of Trust" and asks for the key piece of evidence that led to the arrest of Horace Danby.
Step 2: Detailed Explanation:
Horace Danby was a meticulous thief who always wore gloves to avoid leaving fingerprints. However, during the robbery at Shotover Grange, he is tricked by a young lady (who is also a thief) into believing she is the owner of the house. She asks him to open the safe for her as she has forgotten the combination. Deceived by her, Horace takes off his gloves to open the safe. He then leaves without the jewels. When the robbery is reported, the police find his fingerprints all over the room and on the safe, which serves as conclusive evidence to arrest him.
Step 3: Final Answer:
The crucial evidence against Horace was his fingerprints. The correct option is (C).