Answer
$B$renda committed the crime.
Work Step by Step
We are told that exactly one of these people committed the crime, and exactly one (the guilty party) is a knight.
We look at the three cases to determine who the knight is.
If Amy were the knight, then her protestations of innocence would be true, but that cannot be since we know that the knight is guilty.
If Claire were the knight, then her statement that Brenda is not normal is true; and since Brenda cannot be the knight in this situation, Brenda must be a knave.
That means that Brenda is lying when she says that Amy was telling the truth; therefore Amy is lying.
This means that Amy is guilty, but that cannot be since Amy isn't the knight.
So Brenda must be the knight.
Amy is an innocent normal who is telling the truth when she says she is innocent;
Brenda is telling the truth when she says that Amy is telling the truth, and Claire is a normal who is telling the truth when she says that Brenda is not normal.
So $B$renda committed the crime.
