Convert the sentence provided below from its passive voice structure to an active voice structure:

Ancient artifacts are believed to have been quietly removed from the location by trespassers before the audit.

The correct option is: "Trespassers are believed to have quietly removed ancient artifacts before the audit."

To convert the passive sentence to active voice while maintaining the original meaning and complex verb structure, let's break down the components:

1. Identify the passive structure:
In the original passive sentence, "Ancient artifacts are believed to have been quietly removed from the location by trespassers before the audit," the subject ("Ancient artifacts") receives the action, and the agent performing the main action ("trespassers") is introduced by the preposition "by."

2. Determine the main verb and infinitival verb phrase:
- The reporting verb is present tense passive: "are believed".
- The action verb phrase is a perfect passive infinitive: "to have been quietly removed".

3. Transform into active voice:
- Shift the agent of the action ("trespassers") into the subject position of the infinitival clause.
- Retain the main reporting structure ("are believed").
- Change the passive perfect infinitive ("to have been quietly removed") to an active perfect infinitive ("to have quietly removed").

Putting it all together, the active form is: "Trespassers are believed to have quietly removed ancient artifacts before the audit."