Explanation:

- Option 1: the forged signature

There is no grammatical error here. This phrase is correctly formed as the object of comparison.

- Option 2: to the original

This segment includes a subtle grammatical error. In standard formal English, the correct preposition for direct comparison is "with" not "to." It should read “The detective compared the forged signature with the original.”

- Option 3: The detective compared

This phrase is grammatically correct. It sets the subject and action for the sentence.

- Option 4: No error

This option suggests the sentence is entirely correct, which isn't accurate due to the error identified in Option 2.

Answer: Option 2: to the original